
\section{Skipping Or Changing Coordinates -- Filters}
\label{sec:filters}

\PGFPlots{} offers filters. A filter expects a (numeric) input coordinate and
is allowed to modify the coordinate or throw it away. Filters can either
operate on individual coordinates or on all simultaneously. One main
application is to skip coordinates, i.e.\@ to install a filter which causes the
coordinate to be skipped. This works by assigning special ``this coordinate
does not exist'' values instead of the original values. Valid choices for
``this coordinate does not exist'' are defined using |unbounded coords| as
described in Section~\ref{pgfplots:interrupt} about Interrupted Plots, namely
|empty line| or |unbounded coords|.

See also Section~\ref{sec:transformation:interaction} for different types of
transformations and their interaction.

\begin{pgfplotsxyexpressionkeylist}{\x\space filter}
    Installs a coordinate filter which allows to modify the current value of a
    \emph{single} coordinate.

    The argument \emph{math expression} is a math expression which contains
    |x|, |y|, or |z|.
    %
\begin{codeexample}[]
\begin{tikzpicture}
\begin{axis}[grid=major]
    \addplot+ [
        unbounded coords=jump,
        x filter/.expression={x+1},
        y filter/.expression={y==3 ? nan : y},
    ] table {
        x   y
        1   2
        1.5 2.5
        2   3
        2.5 3.5
        3   4
    };
\end{axis}
\end{tikzpicture}
\end{codeexample}

    The |x filter| is evaluated first. It can depend on |x|, |y|, and |z| whose
    values are the ``prepared'' coordinates: values which have been found after
    applying |x coord trafo| and any logarithms (for logarithmic axes).

    The |y filter| is evaluated as next. It can depend on |x| which is the
    result of |x filter|. It can also depend on |y| and |z| which have the same
    value as discussed in the previous paragraph.

    The |z filter| is evaluated as last. It can depend on |x| and |y| which are
    result of their respective filters. It can also depend on |z| which is the
    plain $z$ coordinate (as discussed for |x filter|).

    Note that filter values can contain expandable material, including
    |\thisrow| for tables:
    %
\begin{codeexample}[]
\begin{tikzpicture}
\begin{axis}
    \addplot+[
        x filter/.expression={
            \thisrow{W} == 42 ? 0.2 : x
        },
    ] table {
        x y     W
        0 0     41
        0.5 0.5 42
        1 1     43
    };
\end{axis}
\end{tikzpicture}
\end{codeexample}
    %
    Defining filters by math expression is actually a special case of
    |x filter|, see below. As a consequence, the \meta{expression} may contain
    |\pgfmathresult| which is the unmodified input value (see the specification
    of |x filter| below).
\end{pgfplotsxyexpressionkeylist}

\begin{pgfplotsxyappendexpressionkeylist}{\x\space filter}
    These are variants of |x filter/.expression| which allow to \emph{append}
    another coordinate filter. In this case, you can have different filters for
    the same coordinate, and each is based on the output of the previous
    filter:
    %
\begin{codeexample}[]
\begin{tikzpicture}
\begin{axis}
    \addplot+[
        y filter/.append expression={
            abs(x-1) < 1e-4 ? 1.5 : \pgfmathresult
        },
        y filter/.append expression={
            abs(x-3) < 1e-4 ? NaN : \pgfmathresult
        },
    ] table {
        x y
        0 0
        1 1
        2 2
        3 3
        4 4
    };
\end{axis}
\end{tikzpicture}
\end{codeexample}
    %
    The example installs two |y filter|s: the first checks if $x = 1$ and
    defines $y=1.5$ in this case, otherwise it tells the filter to ``use the
    value of $y$ as it was before the filter''. The second filter checks if $x
    = 3$ and assigns the value ``Not a Number'' if that is the case (which
    discards the coordinate). Otherwise it tells the filter to ``use the value
    of $y$ as it was before the filter'', which is the value generated by the
    \emph{first} filter.

    As you see, the special macro ``|\pgfmathresult|'' always yields the
    unmodified input value. This is a property of the underlying |x filter|
    functionality.

    Note that the example could be rephrased by replacing the first
    ``|\pgfmathresult|'' by ``|y|''. But in order to simplify copy/paste, it
    makes sense to formulate incremental filters in a way which is always
    incremental.

    Installing |x filter/.append expression| is the same as
    |x filter/.expression| if there was no other filter in place beforehand.
    The ``append'' functionality is applied even if there is some customized
    |x filter| in place (for example one installed with |x filter/.code| as
    outlined below). Thus, ``append'' has the semantics ``append a further
    coordinate filter'' rather than ``append another expression to an existing
    expression''. The only requirement is that a previous filter must not
    replace a coordinate by an empty value. Filters which silently discard the
    value and return an empty one must be appended after |.append expression|.
\end{pgfplotsxyappendexpressionkeylist}

\begin{pgfplotsxycodekeylist}{\x\ filter,filter point}
    The code keys |x filter| and |y filter| are advanced variants of
    |x filter/.expression| and its variants. They allow to modify \emph{single}
    coordinate values, but can execute arbitrary \TeX{} code. A coordinate
    filter gets an input coordinate as |#1| (on input, the same value is stored
    in |\pgfmathresult|), applies some operation and writes the result into the
    macro |\pgfmathresult|. If |\pgfmathresult| is empty afterwards, the
    coordinate is discarded. You can also set |\pgfmathresult| to |nan| or
    |inf| in which case the coordinate can be either discarded (if
    |unbounded coords=discard| is set) or the plot can be interrupted (the case
    |unbounded coords=jump|).

    The |filter point/.code| filter allows filtering depending on all
    components forming a complete point ($x$, $y$ and $z$); it is described
    below.

    It is allowed that filters do not change |\pgfmathresult|. In this case,
    the unfiltered coordinate will be used.

    Coordinate filters are useful in automatic processing system, where
    \PGFPlots{} is used to display automatically generated plots. You may not
    want to filter your coordinates by hand, so these options provide a tool to
    do this automatically.

    The following filter adds $0.5$ to every $x$ coordinate.
    %
\begin{codeexample}[]
\begin{tikzpicture}
\begin{axis}[
    % equivalent to 'x filter/.expression={x+0.5}'
    x filter/.code={
        \pgfmathadd{#1}{0.5},
    },
]
    \addplot coordinates {
        (4,0)
        (6,1)
    };
\end{axis}
\end{tikzpicture}
\end{codeexample}
    %
    Please refer to~\cite[pgfmath manual]{tikz} for details about the math
    engine of \PGF{}. Whenever possible, math-based filters should be formulated
    using |x filter/.expression| as that reduces the complexity and ensures the
    best precision (it also works with |lua backend|).

    During evaluation of the filter, the macro |\coordindex| contains the
    number of the current coordinate (starting with~$0$). Thus, the following
    filter discards all coordinates after the $5$th and before the $10$th.
    %
\begin{codeexample}[]
\begin{tikzpicture}
\begin{axis}[
    samples=20,
    % equivalent to
    % x filter/.expression={
    %   \coordindex>4 && \coordindex<11 ? nan : x
    % },
    x filter/.code={
        \ifnum\coordindex>4
            \ifnum\coordindex<11
                \def\pgfmathresult{}
            \fi
        \fi
    },
]
    \addplot {x^2};
\end{axis}
\end{tikzpicture}
\end{codeexample}
    %
    There is also a style key which simplifies selection by index, see below.

    \PGFPlots{} invokes the filter with argument |#1| set to the input
    coordinate. For $x$ filters, this is the $x$ coordinate as it is specified
    to |\addplot|, for $y$ filters it is the $y$ coordinate.

    If the corresponding axis is logarithmic, |#1| is the \emph{logarithm} (see
    |log basis x| and its variants) of the coordinate as a real number, for
    example |#1=4.2341|. In case the logarithm was undefined, the argument will
    be empty.

    The arguments to coordinate filters are minimally preprocessed: first, for
    logarithmic axes, the \emph{log} of the argument is supplied.  Second, any
    high level coordinate maps like |x coord trafo| (which may be used to map
    dates to numbers or string to numbers or so) are applied. In consequence,
    the |#1| argument is supposed to be a number. No further transformation has
    been applied.

    Occasionally, it might be handy to get the ``raw'', completely unprocessed
    input coordinate as it has been reported by the coordinate input routine.
    This unprocessed data is available in the three math parser constants
    \declareandlabel{rawx}, \declareandlabel{rawy} and \declareandlabel{rawz}.
    All these values are ready for use in filters (and some other methods
    influence plots as well). Note that |rawy| is to be used like a function
    without arguments, i.e.\@ filters can employ it where math parsing is done.

    An application could be to filter log values based on the normal scale:
    %
\begin{codeexample}[]
\begin{tikzpicture}
\begin{semilogyaxis}
    \addplot+ [
        restrict expr to domain={rawy}{1e0:1.5e1},
    ] {exp(x)};
\end{semilogyaxis}
\end{tikzpicture}
\end{codeexample}
    %
    \noindent The preceding example uses |rawy| to throw all samples outside of
    the range $[1,15]$ away.

    If key filters are invoked for |\addplot table|, access to the current row's
    data can be achieved using |\thisrow|\marg{column name} (and its variants).
    This includes all columns of the table.

    The |filter point| key is more technical. It doesn't take an argument: its
    arguments are given in terms of the |pgfkeys| variables |/data point/x|,
    |/data point/y| and |/data point/z|. It may change its coordinates using
    |\pgfkeyssetvalue{/data point/x}|\marg{new value}; access to variables can
    be achieved with |\pgfkeysvalueof{/data point/x}| or, if the argument shall
    be written into a macro, with |\pgfkeysgetvalue|. This filter is evaluated
    after the other ones.

    Note that you can provide different |x filter|/|y filter| arguments to each
    |\addplot| command. It seems there are only problems with the `|#1|'
    argument, and I haven't yet found out why. Please use |\pgfmathresult| in
    place of |#1| if you provide |\addplot[x filter/.code={...}]|.

    Note that coordinate filtering is also available for |mesh|, |surf|, and
    |patch| plots\index{mesh!Coordinate filtering}\index{surf!Coordinate filtering}.
    In this context, a |patch type| is drawn if and only if all its vertices
    have bounded coordinates. In other words: if one vertex of, say, a
    rectangle has been filtered away, the entire rectangle will be omitted.
    Coordinate filtering for |mesh| and |surf|ace plots has a further special
    requirement: the default for such plots is |mesh input=lattice|. If a
    coordinate filter silently discards a coordinate, the lattice will break
    and \PGFPlots{} will become confused. Consequently, coordinate filtering
    for |mesh| and |surf|ace plots always needs |unbounded coords=jump|, and
    any point which is filtered away should receive the value |nan| instead of
    an empty string (since empty strings will \emph{always} be discarded even
    in presence of |unbounded coords=jump|). Please see the reference
    documentation of |unbounded coords| and the example therein on
    page~\pageref{pgfplots:interrupt} for details about coordinate filtering
    and three dimensional plots.
\end{pgfplotsxycodekeylist}

\begin{pgfplotscodekey}{pre filter}
    Applied before |x filter|, |y filter|, and |z filter|.
\end{pgfplotscodekey}

\begin{stylekey}{/pgfplots/skip coords between index=\marg{begin}\marg{end}}
    A style which appends an |x filter| which discards selected coordinates.
    The selection is done by index where indexing starts with~$0$, see
    |\coordindex|. Every coordinate with index $\meta{begin} \le i <
    \meta{end}$ will be skipped.
    %
\begin{codeexample}[]
\begin{tikzpicture}
\begin{axis}[
    samples=20,
    skip coords between index={5}{11},
    skip coords between index={15}{18},
]
    \addplot {x^2};
\end{axis}
\end{tikzpicture}
\end{codeexample}

    \paragraph{Technical note:}

    this style usually applies to $x$ coordinates (i.e.\@ it counts $x$
    coordinates). In case you want to apply it to something like |hist/data| or
    |quiver/u|, you can
    %
    \begin{enumerate}
        \item append an asterisk `|*|' to the style's name and
        \item provide the target coordinate's name as first argument.
    \end{enumerate}
    %
    For example, |skip coords between index*={hist/data}{2}{4}| applies to
    |hist/data|.
\end{stylekey}

\begin{pgfplotskey}{each nth point=\marg{integer}}
    A style which appends an |x filter| which discards all but each $n$th input
    coordinate.\index{Downsampling}

    This downsampling works fairly well. It can be used to reduce a huge amount
    of coordinates from an input file. In this case, you should also set
    |filter discard warning=false| to avoid repeated notifications about
    skipped coordinates and |unbounded coords=discard| such that \PGFPlots{}
    should silently forget any discarded points (rather than generated
    interrupted plots).

    Note that there is also a |mark repeat| style which applies the same
    operation to plot marks only.


    \paragraph{Technical note:}

    this style usually applies to $x$ coordinates (i.e.\@ it counts $x$
    coordinates). In case you want to apply it to something like |hist/data| or
    |quiver/u|, you can
    %
    \begin{enumerate}
        \item append an asterisk `|*|' to the style's name and
        \item provide the target coordinate's name as first argument.
    \end{enumerate}
    %
    For example, |each nth point*={hist/data}{2}{4}| applies to |hist/data|.
\end{pgfplotskey}

\begin{pgfplotsxykeylist}{%
    restrict \x\space to domain=\meta{min}:\meta{max},
    restrict \x\space to domain*=\meta{min}:\meta{max}%
}
\label{key:restrict:x:to:domain}
    These keys append $x$ (or $y$ or $z$) coordinate filters to restrict the
    respective coordinate to a domain.

    \tikzset{external/figure name/.add={}{restrictodomain_}}%

    The versions without star (like |restrict x to domain|) will assign the
    value |-inf| if the coordinate is below \meta{min} and |+inf| if the
    coordinate is above \meta{max}. The starred versions (like
    |restrict x to domain*|) will truncate coordinates to $[\hbox{\meta{min}},
    \hbox{\meta{max}}]$, i.e.\@ they assign the value \meta{min} if the
    coordinate falls outside of the lower limit and \meta{max} if the value
    falls outside of the upper limit.

    For logarithmic axes, \meta{min} and \meta{max} are \emph{logs} of the
    respective values. A variant which uses the non-logarithmic number might be
    to use |restrict expr to domain={\pgfmathrawx}|\marg{min}\marg{max}.

    The non-starred versions also set |unbounded coords=jump| which leads to
    interrupted plots.
    %
\pgfplotsexpensiveexample
\begin{codeexample}[]
\begin{tikzpicture}
    \begin{axis}[
        restrict y to domain=-10:10,
        samples=1000,
        % some fine-tuning for the display:
        width=10cm, height=210pt,
        xmin=-4.7124, xmax=4.7124,
        xtick={-4.7124,-1.5708,...,10},
        xticklabels={$-\frac32 \pi$,$-\pi/2$,$\pi/2$,$\frac32 \pi$},
        axis x line=center,
        axis y line=center,
    ]
        \addplot [blue] gnuplot [id=tangens,domain=-1.5*pi:1.5*pi] {tan(x)};
        \legend{$\tan(x)$}
    \end{axis}
\end{tikzpicture}
\end{codeexample}
\end{pgfplotsxykeylist}

\begin{pgfplotskeylist}{%
    restrict expr to domain=\marg{expression}\marg{\meta{min}:\meta{max}},
    restrict expr to domain*=\marg{expression}\marg{\meta{min}:\meta{max}}%
}
    Appends an $x$ coordinate filter which sets the $x$ coordinate to |-inf| if
    the \meta{expression} evaluates to something less than \meta{min} and to
    |inf| if \meta{expression} evaluates to something larger than \meta{max}.

    The starred variant, |restrict to domain*| assigns \meta{min} if
    \meta{expression} is less then the lower limit and \meta{max} if it is
    larger than the upper limit.

    The non-starred version also sets |unbounded coords=jump| which leads to
    interrupted plots.

    In contrast to |restrict x to domain|, \meta{expression} can depend on
    anything which is valid during |\addplot|, in particular |\coordindex| or
    table columns (|\thisrow|\marg{column name} and friends). The expression
    doesn't need to depend on $x$ at all.
\end{pgfplotskeylist}

\begin{pgfplotskey}{%
    @restrict to domain=\marg{filter name}\marg{expression}\marg{\meta{min}:\meta{max}}\mchoice{0,1}%
}
    A low-level (technical) key which allows to apply the |restrict * to ...|
    features also to something like |hist/data|.

    For example, |@restrict to domain={hist/data}{}{0:1}{0}| applies the
    domain-restriction to the histogram-input |hist/data|. The final `|0|'
    means that it works in a similar way as the key |restrict x to domain=0:1|,
    i.e.\@ it skips everything which is outside of $[0,1]$.  In a similar way,
    |@restrict to domain={hist/data}{}{0:1}{1}| applies the functionality of
    |restrict x to domain*=0:1| to |hist/data|: it truncates values outside of
    $[0,1]$ to the domain's end-points.

    The \meta{filter name} is expected to be a coordinate name like |x|, |y|,
    |z| (or |hist/data|).

    The \meta{expression} configures an expression which will be used rather
    than the value of \meta{filter name}. It can be empty.

    The \meta{min}\texttt{:}\meta{max} are as described above.

    If the last argument is |1|, any coordinate outside of the allowed domain
    will take the domain boundary as value. If it is |0|, such a coordinate
    will get either |inf| or |-inf|.
\end{pgfplotskey}

\begin{pgfplotskey}{filter discard warning=\mchoice{true,false} (initially true)}
    Issues a notification in your logfile whenever coordinate filters discard
    coordinates.
\end{pgfplotskey}

You can find somewhat more on coordinate filtering in
Section~\ref{pgfplots:interrupt}: ``Interrupted Plots''.
