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authorNorbert Preining <norbert@preining.info>2019-09-02 13:46:59 +0900
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+% Copyright 2008 by Till Tantau
+%
+% This file may be distributed and/or modified
+%
+% 1. under the LaTeX Project Public License and/or
+% 2. under the GNU Free Documentation License.
+%
+% See the file doc/generic/pgf/licenses/LICENSE for more details.
+
+
+\section{Creating Data Visualizations}
+\label{section-dv-main}
+\label{section-dv-main-setup}
+
+\subsection{Overview}
+
+The \todosp{why two labels? The first doesn't seem to be used.} present section
+explains how a data visualization is created in \tikzname. For this, you need
+to include the |datavisualization| library and then use the command
+|\datavisualization| whose syntax is explained in the rest of the present
+section. This command is part of the following library:
+
+\begin{tikzlibrary}{datavisualization}
+ This library must be loaded if you wish to use the |\datavisualization|
+ command. It defines all styles needed to create basic data visualizations;
+ additional, more specialized libraries need to be loaded for more advanced
+ features.
+\end{tikzlibrary}
+
+In order to visualize, you basically need to do three things:
+%
+\begin{enumerate}
+ \item You need to select what kind of plot you would like to have (a
+ ``school book plot'' or a ``scientific 2d plot'' or a ``scientific
+ spherical plot'' etc.). This is done by passing an option to the
+ |\datavisualization| command that selects this kind of plot.
+ \item You need to provide data points, which is done using the |data|
+ command.
+ \item Additionally, you can add options that give you more fine-grained
+ control over the way the visualization will look. You can configure the
+ number of ticks and grid lines, where the labels are placed, the
+ colors, or the fonts. Indeed, since the data visualization engine
+ internally uses \tikzname-styles, you can have extremely fine-grained
+ control over how a plot will look like.
+\end{enumerate}
+
+The syntax of the |\datavisualization| command is designed in such a way that
+if you only need to provide very few options to create plots that ``look good
+by default''.
+
+This section is structured as follows: First, the philosophy behind concepts
+like ``data points'', ``axes'', or ``visualizers'' is explained. Each of these
+concepts is further detailed in later section. Then, the syntax of the
+|\datavisualization| command is covered. The reference sections explain which
+predefined plot kinds are available.
+
+
+\subsection{Concept: Data Points and Data Formats}
+
+As explained in Section~\ref{section-dv-intro-data-points}, data points are the
+basic entities that are processed by the data visualization engine. In order to
+specify data points, you use the |data| command, whose syntax is explained in
+more detail in Section~\ref{section-dv-data-syntax}. The |data| command allows
+you to either specify points ``inline'', directly inside your \TeX-file; or you
+can specify the name of file that contains the data points.
+
+\medskip
+\textbf{Specifying data points.}
+Data points can be formatted in different ways. For instance, in the so called
+\emph{comma separated values} format, there is one line for each data point and
+the different attributes of a data point are separated by commas. Another
+common format is to specify data points using the so called \emph{key--value}
+format, where on each line the different attributes of a data point are set
+using a comma-separated list of strings of the form |attribute=value|.
+
+Here are two examples, where similar data is given in different formats:
+%
+\begin{codeexample}[]
+\begin{tikzpicture}
+ \datavisualization [school book axes, visualize as smooth line]
+ data {
+ x, y
+ -1.5, 2.25
+ -1, 1
+ -.5, .25
+ 0, 0
+ .5, .25
+ 1, 1
+ 1.5, 2.25
+ };
+\end{tikzpicture}
+\end{codeexample}
+
+\begin{codeexample}[]
+\begin{tikzpicture}
+ \datavisualization [school book axes, visualize as smooth line]
+ data [format=function] {
+ var x : interval [-1.5:1.5] samples 7;
+ func y = \value x*\value x;
+ };
+\end{tikzpicture}
+\end{codeexample}
+
+In the first example, no format needed to be specified explicitly since the
+default format is the one used for the data following the |data| keyword: A
+list of comma-separated values, where each line represents a data point.
+
+\medskip
+\textbf{Number accuracy.}\label{section-dv-expressions}
+Data visualizations typically demand a much higher accuracy and range of values
+than \TeX\ provides: \TeX\ numbers are limited to 13 bits for the integer part
+and 16 bits for the fractional part. Because of this, the data visualization
+engine does not use \pgfname's standard representation of numbers and \TeX\
+dimensions and is does not use the standard parser when reading numbers in a
+data point. Instead, the |fpu| library, described in
+Section~\ref{section-library-fpu}, is used to handle numbers.
+
+This use of the |fpu| library has several effects that users of the data
+visualization system should be aware of:
+%
+\begin{enumerate}
+ \item You can use numbers like |100000000000000| or |0.00000000001| in a
+ data points.
+ \item Since the |fpu| library does not support advanced parsing, you
+ currently \emph{cannot} write things like |3+2| in a data point number.
+ This will result in an error.
+ \item However, there is a loop-hole: If a ``number'' in a data point starts
+ with a parenthesis, the value between the parentheses \emph{is} parsed
+ using the normal parser:
+ %
+ \begin{itemize}
+ \item |100000| is allowed.
+ \item |2+3| yields an error.
+ \item |(2+3)| is allowed and evaluates to |5|.
+ \item |(100000)| yields an error since $100\,000$ is beyond the
+ normal parser's precision.
+ \end{itemize}
+ %
+ The bottom line is that any normal calculations should be set inside
+ round parentheses, while large numbers should not be surrounded by
+ parentheses. Hopefully, in the future, these restrictions will be
+ lifted.
+\end{enumerate}
+
+Section~\ref{section-dv-formats} gives an in-depth coverage of the available
+data formats and explains how new data formats can be defined.
+
+
+\subsection{Concept: Axes, Ticks, and Grids}
+
+Most plots have two or three axes: A horizontal axis usually called the
+$x$-axis, a vertical axis called the $y$-axis, and possibly some axis pointing
+in a sloped direction called the $z$-axis. Axes are usually drawn as lines with
+\emph{ticks} indicating interesting positions on the axes. The data
+visualization engine gives you detailed control over where these ticks are
+rendered and how many of them are used. Great care is taken to ensure that the
+position of ticks are chosen well by default.
+
+From the point of view of the data visualization engine, axes are a somewhat
+more general concept than ``just'' lines that point ``along'' some dimension:
+The data visualization engine uses axes to visualize any change of an attribute
+by varying the position of data points in the plane. For instance, in a polar
+plot, there is an ``axis'' for the angle and another ``axis'' for the distance
+if the point from the center. Clearly these axes vary the position of data
+points in the plane according to some attribute of the data points; but just as
+clearly they do not point in any ``direction''.
+
+A great benefit of this approach is that the powerful methods for specifying
+and automatic inference of ``good'' positions for ticks or grid lines apply to
+all sorts of situations. For instance, you can use it to automatically put
+ticks and grid lines at well-chosen angles of a polar plot.
+
+Typically, you will not need to specify axes explicitly. Rather, predefined
+styles take care of this for you:
+%
+\begin{codeexample}[]
+\begin{tikzpicture}
+ \datavisualization [
+ scientific axes,
+ x axis={length=3cm, ticks=few},
+ visualize as smooth line
+ ]
+ data [format=function] {
+ var x : interval [-1.5:1.5] samples 7;
+ func y = \value x*\value x;
+ };
+\end{tikzpicture}
+\end{codeexample}
+
+\begin{codeexample}[]
+\begin{tikzpicture}
+ \datavisualization [
+ scientific axes=clean,
+ x axis={length=3cm, ticks=few},
+ all axes={grid},
+ visualize as smooth line
+ ]
+ data [format=function] {
+ var x : interval [-1.5:1.5] samples 7;
+ func y = \value x*\value x;
+ };
+\end{tikzpicture}
+\end{codeexample}
+
+Section~\ref{section-dv-axes} explains in more detail how axes, ticks, and grid
+lines can be chosen and configured.
+
+
+\subsection{Concept: Visualizers}
+
+Data points and axes specify \emph{what} is visualized and \emph{where}. A
+\emph{visualizer} specifies \emph{how} they are visualized. One of the most
+common visualizers is a \emph{line visualizer} which connects the positions of
+the data points in the plane using a line. Another common visualizer is the
+\emph{scatter plot visualizer} where small marks are drawn at the positions of
+the data points. More advanced visualizers include, say, box plot visualizers
+or pie chart visualizers.
+%
+\begin{codeexample}[]
+\begin{tikzpicture}
+ \datavisualization [
+ scientific axes=clean,
+ x axis={length=3cm, ticks=few},
+ visualize as smooth line
+ ]
+ data [format=function] {
+ var x : interval [-1.5:1.5] samples 7;
+ func y = \value x*\value x;
+ };
+\end{tikzpicture}
+\end{codeexample}
+%
+\begin{codeexample}[]
+\begin{tikzpicture}
+ \datavisualization [
+ scientific axes=clean,
+ x axis={length=3cm, ticks=few},
+ visualize as scatter
+ ]
+ data [format=function] {
+ var x : interval [-1.5:1.5] samples 7;
+ func y = \value x*\value x;
+ };
+\end{tikzpicture}
+\end{codeexample}
+
+Section~\ref{section-dv-visualizers} provides more information on visualizers
+as well as reference lists.
+
+
+\subsection{Concept: Style Sheets and Legends}
+
+A single data visualizations may use more than one visualizer. For instance, if
+you wish to create a plot containing several lines, a separate visualizer is
+used for each line. In this case, two problems arise:
+%
+\begin{enumerate}
+ \item You may wish to make it easy for the reader to differentiate between
+ the different visualizers. For instance, one line should be black,
+ another should be red, and another blue. Alternatively, you might wish
+ one line to be solid, another to be dashed, and a third to be dotted.
+
+ Specifying such styles is trickier than one might expect; experience
+ shows that many plots use ill-chosen and inconsistent styling. For this
+ reason, the data visualization introduces the notion of \emph{style
+ sheets} for visualizers and comes with some well-designed predefined
+ style sheets.
+ \item You may wish to add information concerning what the different
+ visualizers represent. This is typically done using a legend, but it is
+ even better to add labels directly inside the visualization. Both
+ approaches are supported.
+\end{enumerate}
+
+An example where three functions are plotted and a legend is added is shown
+below. Two style sheets are used so that \emph{both} the coloring and the
+dashing is varied.
+%
+\begin{codeexample}[]
+\begin{tikzpicture}[baseline]
+ \datavisualization [ scientific axes=clean,
+ y axis=grid,
+ visualize as smooth line/.list={sin,cos,tan},
+ style sheet=strong colors,
+ style sheet=vary dashing,
+ sin={label in legend={text=$\sin x$}},
+ cos={label in legend={text=$\cos x$}},
+ tan={label in legend={text=$\tan x$}},
+ data/format=function ]
+ data [set=sin] {
+ var x : interval [-0.5*pi:4];
+ func y = sin(\value x r);
+ }
+ data [set=cos] {
+ var x : interval [-0.5*pi:4];
+ func y = cos(\value x r);
+ }
+ data [set=tan] {
+ var x : interval [-0.3*pi:.3*pi];
+ func y = tan(\value x r);
+ };
+\end{tikzpicture}
+\end{codeexample}
+
+Section~\ref{section-dv-style-sheets} details style sheets and legends.
+
+
+\subsection{Usage}
+\label{section-dv-data-syntax}
+
+Inside a \tikzname\ picture you can use the |\datavisualization| command to
+create a data visualization. You can use this command several times in a
+picture to create pictures containing multiple data visualizations.
+
+\begin{command}{\datavisualization\opt{\oarg{data visualization options}}\meta{data specification}|;|}
+ This command is available only inside a |{tikzpicture}| environment.
+
+ The \meta{data visualization options} are used to configure the data
+ visualization, that is, how the data is to be depicted. The options are
+ executed with the path prefix |/tikz/data visualization|. This means that
+ normal \tikzname\ options like |thin| or |red| cannot be used here. Rather,
+ a large number of options specific to data visualizations are available.
+
+ As a minimum, you should specify at least two options: First, you should
+ use an option that selects an axis system that is appropriate for your
+ plot. Typical possible keys are |school book axes| or |scientific axes|,
+ detailed information on them can be found in Section~\ref{section-dv-axes}.
+
+ Second, you use an option to select \emph{how} the data should be
+ visualized. This is done using a key like |visualize as line| which will,
+ as the name suggests, visualize the data by connecting data points in the
+ plane using a line. Similarly, |visualize as smooth cycle| will try to fit
+ a smooth cycle through the data points. Detailed information on possible
+ visualizers can be found in Section~\ref{section-dv-visualizers}.
+
+ Following these options, the \meta{data specification} is used to provide
+ the actual to-be-visualized data. The syntax is somewhat similar to
+ commands like |\path|: The \meta{data specification} is a sequence of
+ keywords followed by local options and parameters, terminated with a
+ semicolon. (Indeed, like for the |\path| command, the \meta{data
+ visualizers options} need not be specified at the beginning, but additional
+ option surrounded by square brackets may be given anywhere inside the
+ \meta{data specification}.)
+
+ The different possible keywords inside the \meta{data specification} are
+ explained in the following.
+\end{command}
+
+\begin{datavisualizationoperation}{data}{\opt{\oarg{options}}\opt{\marg{inline data}}}
+ This command is used to specify data for the data visualization. It can be
+ used several times inside a single visualization and each time the
+ to-be-read data may have a different format, but the data will be
+ visualized as if it have been specified inside a single |data| command.
+
+ The behaviour of the |data| command depends on whether the \meta{inline
+ data} is present. If it is not present, the \meta{options} must be used to
+ specify a source file from which the data is read; if the \meta{inline
+ data} is present no file will be used, instead the data should directly
+ reside inside the \TeX-file and be given between the curly braces
+ surrounding the \meta{inline data}.
+
+ The \meta{options} are executed with the prefix |/pgf/data|. The following
+ options are always available:
+ %
+ \begin{key}{/pgf/data/read from file=\meta{filename} (initially \normalfont empty)}
+ If you set the |source| attribute to a non-empty \meta{filename}, the
+ data will be read from this file. In this case, no \meta{inline data}
+ may be present, not even empty curly braces should be provided.
+ %
+\begin{codeexample}[code only]
+\datavisualization ...
+ data [read from file=file1.csv]
+ data [read from file=file2.csv];
+\end{codeexample}
+ %
+ The other way round, if |read from file| is empty, the data must
+ directly follow as \meta{inline data}.
+ %
+\begin{codeexample}[code only]
+\datavisualization ...
+ data {
+ x, y
+ 1, 2
+ 2, 3
+ };
+\end{codeexample}
+ \end{key}
+ %
+ The second important key is |format|, which is used to specify the data
+ format:
+ %
+ \begin{key}{/pgf/data/format=\meta{format} (initially table)}
+ Use this key to locally set the format used for parsing the data, see
+ Section~\ref{section-dv-formats} for a list of predefined formats.
+
+ The default format is the |table|-format, also known as
+ ``comma-separated values''. The first line contains names of attributes
+ separated by commas, all following lines constitute a data point where
+ the attributes are given by the comma-separated values in that line.
+ \end{key}
+
+
+ \medskip
+ \textbf{Presetting attributes.}
+ Normally, the inline data or the external data contains for each data point
+ the values of the different attributes. However, sometimes you may also
+ wish to set an attribute to a fixed value for all data points of a data
+ set. Suppose, for instance, that you have to source files
+ |experiment007.csv| and |experiment023.csv| and you would like that for all
+ data points of the first file the attribute |/data point/experiment id| is
+ set to 7 while for the data points of the second file they are set to 23.
+ In this case, you can specify the desired settings using an absolute path
+ inside the \meta{options}. The effect will be local to the current |data|
+ command:
+ %
+\begin{codeexample}[code only]
+\datavisualization...
+ data [/data point/experiment=7, read from file=experiment007.csv]
+ data [/data point/experiment=23, read from file=experiment023.csv];
+\end{codeexample}
+
+\begin{codeexample}[]
+\tikz
+ \datavisualization [school book axes, visualize as line]
+ data [/data point/x=1] {
+ y
+ 1
+ 2
+ }
+ data [/data point/x=2] {
+ y
+ 2
+ 0.5
+ };
+\end{codeexample}
+
+
+ \medskip
+ \textbf{Setting options for multiple |data| commands.}
+ You may wish to generally set the format once and for all. This can be done
+ by using the following key:
+ %
+ \begin{stylekey}{/tikz/every data}
+ This key is executed for every |data| command.
+ \end{stylekey}
+
+ Another way of passing options to multiple |data| commands is to use the
+ following facility: Whenever an option with the path
+ |/tikz/data visualization/data| is used, the path will be remapped to
+ |/pgf/data|. This means, in particular, that you can pass an option like
+ |data/format=table| to the |\datavisualization| command to set the data
+ format for all |data| commands of the data visualization.
+
+
+ \medskip
+ \textbf{Parsing inline data.}
+ When you specify data inline, \TeX\ needs to read the data
+ ``line-by-line'', while \TeX\ normally largely ignores end-of-line
+ characters. For this reason, the data visualization system temporarily
+ changes the meaning of the end-of-line character. This is only possible if
+ \TeX\ has not already processed the data in some other way (namely as the
+ parameter to some macro).
+
+ The bottom line is that you cannot use inline data when the whole
+ |\datavisualization| command is passed as a parameter to some macro that is
+ not setup to handle ``fragile'' code. For instance, in a \textsc{beamer}
+ |frame| you need to add the |fragile| option when a data visualization
+ contains inline data.
+
+ The problem does not arise when an external data |source| is specified.
+\end{datavisualizationoperation}
+
+\begin{datavisualizationoperation}{data point}{\opt{\oarg{options}}}
+ This command is used to specify data a single data point. The
+ \meta{options} are simply executed with the path |/data point| and then a
+ data point is created. This means that inside the \meta{options} you just
+ specify the values of all attributes in key--value syntax.
+ %
+\begin{codeexample}[]
+\tikz \datavisualization [school book axes, visualize as line]
+ data point [x=1, y=1] data point [x=1, y=2]
+ data point [x=2, y=2] data point [x=2, y=0.5];
+\end{codeexample}
+ %
+\end{datavisualizationoperation}
+
+\begin{key}{/tikz/data visualization/data point=\meta{options}}
+ This key is the ``key version'' of the previous command. The difference is
+ that this key can be used internally inside styles.
+ %
+\begin{codeexample}[]
+\tikzdatavisualizationset{
+ horizontal/.style={
+ data point={x=#1, y=1}, data point={x=#1, y=2}},
+}
+\tikz \datavisualization
+[ school book axes, visualize as line,
+ horizontal=1,
+ horizontal=2 ];
+\end{codeexample}
+ %
+\end{key}
+
+\begin{datavisualizationoperation}{data group}{\opt{\oarg{options}}\marg{name}\opt{|+=|\marg{data specifications}}}
+ You can store a whole \meta{data specification} in a \emph{data group}.
+ This allows you to reuse data in multiple places without having to write
+ the data to an external file.
+
+ The syntax of this command comes in the following three variants:
+ %
+ \begin{itemize}
+ \item |data group| \opt{\oarg{options}} \marg{name} |=| \marg{data
+ specifications}
+ \item |data group| \opt{\oarg{options}} \marg{name} |+=| \marg{data
+ specifications}
+ \item |data group| \opt{\oarg{options}} \marg{name}
+ \end{itemize}
+ %
+ In the first case, a new data group called \meta{name} is created (an
+ existing data group of the same name will be erased) and the following
+ \meta{data specifications} is stored in this data group. The data group
+ will not be fed to the rendering pipeline, but it is parsed at this point
+ as if it were. The defined data group is defined globally, so you can used
+ it in subsequent visualizations. The \meta{options} are saved with the
+ parsed \meta{data specifications}.
+
+ In the second case, an already existing data group is extended by adding
+ the \meta{data specifications} to it.
+
+ In the third case (detected by noting that the \meta{name} is neither
+ followed by an equal sign nor a plus sign), the contents of the previously
+ defined data group \meta{name} is inserted. The \meta{options} are also
+ executed.
+
+ Let is now first create a data group. Note that nothing is drawn since the
+ ``dummy'' data visualization is empty and used only for the definition of
+ the data group.
+ %
+\begin{codeexample}[]
+\tikz \datavisualization data group {points} = {
+ data {
+ x, y
+ 0, 1
+ 1, 2
+ 2, 2
+ 5, 1
+ 2, 0
+ 1, 1
+ }
+};
+\end{codeexample}
+
+ We can now use this data in different plots:
+ %
+\begin{codeexample}[]
+\tikz \datavisualization [school book axes, visualize as line] data group {points};
+\qquad
+\tikz \datavisualization [scientific axes=clean, visualize as line] data group {points};
+\end{codeexample}
+ %
+\end{datavisualizationoperation}
+
+\begin{datavisualizationoperation}{scope}{\opt{\oarg{options}}\marg{data specification}}
+ Scopes can be used to nest hierarchical data sets. The \meta{options} will
+ be executed with the path |/pgf/data| and will only apply to the data sets
+ specified inside the \meta{data specification}, which may contain |data| or
+ |scope| commands once more:
+ %
+\begin{codeexample}[code only]
+\datavisualization...
+ scope [/data point/experiment=7]
+ {
+ data [read from file=experiment007-part1.csv]
+ data [read from file=experiment007-part2.csv]
+ data [read from file=experiment007-part3.csv]
+ }
+ scope [/data point/experiment=23, format=foo]
+ {
+ data [read from file=experiment023-part1.foo]
+ data [read from file=experiment023-part2.foo]
+ };
+\end{codeexample}
+ %
+\end{datavisualizationoperation}
+
+\begin{datavisualizationoperation}{info}{\opt{\oarg{options}}\marg{code}}
+ This command will execute normal \tikzname\ \meta{code} at the end of a
+ data visualization. The \meta{options} are executed with the normal path
+ |/tikz/|.
+
+ The only difference between this command and just giving the \meta{code}
+ directly following the data visualization is that inside the \meta{code}
+ following an |info| command you still have access to the coordinate system
+ of the data visualization. In sharp contrast, \tikzname\ code given after a
+ data visualization can no longer access this coordinate system.
+ %
+\begin{codeexample}[]
+\begin{tikzpicture}[baseline]
+ \datavisualization [ school book axes, visualize as line ]
+ data [format=function] {
+ var x : interval [-0.1*pi:2];
+ func y = sin(\value x r);
+ }
+ info {
+ \draw [red] (visualization cs: x={(.5*pi)}, y=1) circle [radius=1pt]
+ node [above,font=\footnotesize] {extremal point};
+ };
+\end{tikzpicture}
+\end{codeexample}
+
+ As can be seen, inside a data visualization a special coordinate system is
+ available:
+
+ \begin{coordinatesystem}{visualization}
+ As for other coordinate systems, the syntax is
+ \declare{|(visualization cs:|\meta{list of attribute-value pairs}|)|}.
+ The effect is the following: For each pair
+ \meta{attribute}|=|\meta{value} in the \meta{list} the key
+ |/data point/|\meta{attribute} is set to \meta{value}. Then, it is
+ computed where the resulting data point ``would lie'' on the canvas
+ (however, no data point is passed to the visualizers).
+ \end{coordinatesystem}
+\end{datavisualizationoperation}
+
+\begin{datavisualizationoperation}{info'}{\opt{\oarg{options}}\marg{code}}
+ This command works like |info|, only the \meta{code} will be executed just
+ before the visualization is done. This allows you to draw things
+ \emph{behind} the visualization.
+ %
+\begin{codeexample}[]
+\begin{tikzpicture}[baseline]
+ \datavisualization [ school book axes, visualize as line ]
+ data [format=function] {
+ var x : interval [-0.1*pi:2];
+ func y = sin(\value x r);
+ }
+ info' {
+ \fill [red] (visualization cs: x={(.5*pi)}, y=1) circle [radius=2mm];
+ };
+\end{tikzpicture}
+\end{codeexample}
+ %
+\end{datavisualizationoperation}
+
+\label{section-dv-bounding-box}%
+\begin{predefinednode}{data visualization bounding box}
+ This rectangle node stores a bounding box of the data visualization that is
+ currently being constructed. This node can be useful inside |info| commands
+ or when labels need to be added.
+\end{predefinednode}
+
+\begin{predefinednode}{data bounding box}
+ This rectangle node is similar to |data visualization bounding box|, but it
+ keeps track only of the actual data. The spaces needed for grid lines,
+ ticks, axis labels, tick labels, and other all other information that is
+ not part of the actual data is not part of this box.
+\end{predefinednode}
+
+
+\subsection{Advanced: Executing User Code During a Data Visualization}
+\label{section-dv-user-code}
+
+The following keys can be passed to the |\datavisualization| command and allow
+you to execute some code at some special time during the data visualization
+process. For details of the process and on which signals are emitted when, see
+Section~\ref{section-dv-backend}.
+
+\begin{key}{/tikz/data visualization/before survey=\meta{code}}
+ The \meta{code} is passed to the |before survey| method of the data
+ visualization object and then executed at the appropriate time (see
+ Section~\ref{section-dv-backend} for details).
+
+ The following commands work likewise:
+\end{key}
+%
+\begin{key}{/tikz/data visualization/at start survey=\meta{code}}
+\end{key}
+%
+\begin{key}{/tikz/data visualization/at end survey=\meta{code}}
+\end{key}
+%
+\begin{key}{/tikz/data visualization/after survey=\meta{code}}
+\end{key}
+%
+\begin{key}{/tikz/data visualization/before visualization=\meta{code}}
+\end{key}
+%
+\begin{key}{/tikz/data visualization/at start visualization=\meta{code}}
+\end{key}
+%
+\begin{key}{/tikz/data visualization/at end visualization=\meta{code}}
+\end{key}
+%
+\begin{key}{/tikz/data visualization/after visualization=\meta{code}}
+\end{key}
+
+
+\subsection{Advanced: Creating New Objects}
+
+You will need the following key only when you wish to create new rendering
+pipelines from scratch -- instead of modifying an existing pipeline as you
+would normally do. In the following it is assumed that you are familiar with
+the concepts of Section~\ref{section-dv-backend}.
+
+\begin{key}{/tikz/data visualization/new object=\meta{options}}
+ This key serves two purposes:
+ %
+ \begin{enumerate}
+ \item This method makes it easy to create a new object as part of the
+ rendering pipeline, using \meta{options} to specify arguments
+ rather that directly calling |\pgfoonew|. Since you have the full
+ power of the keys mechanism at your disposal, it is easy, for
+ instance, to control whether or not parameters to the constructor
+ are expanded or not.
+ \item The object is not created immediately, but only just before the
+ visualization starts. This allows you to specify that an object
+ must be created, but the parameter values of for its constructor
+ may depend on keys that are not yet set. A typical application is
+ the creating of an axis object: When you say |scientific axes|, the
+ |new object| command is used internally to create two objects
+ representing these axes. However, keys like |x={length=5cm}| can
+ only \emph{later} be used to specify the parameters that need to be
+ passed to the constructor of the objects.
+ \end{enumerate}
+
+ The following keys may be used inside the \meta{options}:
+ %
+ \begin{key}{/tikz/data visualization/class=\meta{class name}}
+ The class of the to-be-created object.
+ \end{key}
+ %
+ \begin{key}{/tikz/data visualization/when=\meta{phase name} (initially before survey)}
+ This key is used to specify when the object is to be created. As
+ described above, the object is not created immediately, but at some
+ time during the rendering process. You can specify any of the phases
+ defined by the data visualization object, see
+ Section~\ref{section-dv-backend} for details.
+ \end{key}
+ %
+ \begin{key}{/tikz/data visualization/store=\meta{key name}}
+ If the \meta{key name} is not empty, once the object has been created,
+ a handle to the object will be stored in \meta{key name}. If a handle
+ is already stored in \meta{key name}, the object is not created twice.
+ \end{key}
+ %
+ \begin{key}{/tikz/data visualization/before creation=\meta{code}}
+ This code is executed right before the object is finally created. It
+ can be used to compute values that are then passed to the constructor.
+ \end{key}
+ %
+ \begin{key}{/tikz/data visualization/after creation=\meta{code}}
+ This code is executed right after the object has just been created. A
+ handle to the just-created object is available in |\tikzdvobj|.
+ \end{key}
+ %
+ \begin{key}{/tikz/data visualization/arg1=\meta{value}}
+ The value to be passed as the first parameter to the constructor.
+ Similarly, the keys |arg2| to |arg8| specify further parameters passed.
+ Naturally, only as many arguments are passed as parameters are set.
+ Here is an example:
+ %
+\begin{codeexample}[code only]
+\tikzdatavisualizationset{
+ new object={
+ class = example class,
+ arg1 = foo,
+ arg2 = \bar
+ }
+}
+\end{codeexample}
+ %
+ causes the following object creation code to be executed later on:
+ %
+\begin{codeexample}[code only]
+\pgfoonew \tikzdvobj=new example class(foo,\bar)
+\end{codeexample}
+ %
+ Note that you key mechanisms like |.expand once| to pass the value of a
+ macro instead of the macro itself:
+ %
+\begin{codeexample}[code only]
+\tikzdatavisualizationset{
+ new object={
+ class = example class,
+ arg1 = foo,
+ arg2/.expand once = \bar
+ }
+}
+\end{codeexample}
+ %
+ Now, if |\bar| is set to |This \emph{is} it.|\@ at the moment to object
+ is created later on, the following object creation code is executed:
+ %
+\begin{codeexample}[code only]
+\pgfoonew \tikzdvobj=new example class(foo,This \emph{is} it)
+\end{codeexample}
+ \end{key}
+
+ \begin{key}{/tikz/data visualization/arg1 from key=\meta{key}}
+ Works like the |arg1|, only the value that is passed to the constructor
+ is the current value of the specified \meta{key} at the moment when the
+ object is created.
+ %
+\begin{codeexample}[code only]
+\tikzdatavisualizationset{
+ new object={
+ class = example class,
+ arg1 from key = /tikz/some key
+ }
+}
+\tikzset{some key/.initial=foobar}
+\end{codeexample}
+ %
+ causes the following to be executed:
+ %
+\begin{codeexample}[code only]
+\pgfoonew \tikzdvobj=new example class(foobar)
+\end{codeexample}
+ %
+ Naturally, the keys |arg2 from key| to |arg8 from key| are also
+ provided.
+ \end{key}
+
+ \begin{key}{/tikz/data visualization/arg1 handle from key=\meta{key}}
+ Works like the |arg1 from key|, only the key must store an object and
+ instead of the object a handle to the object is passed to the
+ constructor.
+ \end{key}
+\end{key}