


\subsection{Grid Options}
	
\begin{pgfplotsxykeylist}{\x minorgrids=\mchoice{true,false} (initially false),\x majorgrids=\mchoice{true,false} (initially false),grid=\mchoice{minor,major,both,none} (initially false)}
Enables/disables different grid lines. Major grid lines are placed at the normal tick positions (see |xmajorticks|) while minor grid lines are placed at minor ticks (see |xminorticks|). 

This example employs the coordinates defined on page~\pageref{page:plotcoords:src}.
\begin{codeexample}[]
\begin{tikzpicture}
\begin{loglogaxis}[
	xlabel={\textsc{Dof}},
	ylabel={$L_2$ Error},
	grid=major
]
% see above for this macro:
\plotcoords
\end{loglogaxis}
\end{tikzpicture}
\end{codeexample}

\begin{codeexample}[]
\begin{tikzpicture}
\begin{loglogaxis}[
	grid=both,
	tick align=outside,
	tickpos=left]
\addplot coordinates 
	{(100,1e-4) (500,1e-5) (1000,3e-6)};
\addplot coordinates 
	{(100,1e-5) (500,4e-6) (1000,2e-6)};
\end{loglogaxis}
\end{tikzpicture}
\end{codeexample}

Grid lines will be drawn before tick lines are processed, so ticks will be drawn on top of grid lines. You can configure the appearance of grid lines with the styles
\begin{codeexample}[code only]
\pgfplotsset{grid style={help lines}} % modifies the style `every axis grid'
\pgfplotsset{minor grid style={color=blue}} % modifies the style `every minor grid'
\pgfplotsset{major grid style={thick}} %modifies the style `every major grid'
\end{codeexample}
\end{pgfplotsxykeylist}


\subsection{Accessing Axis Coordinates for Annotations}
\label{sec:axis:coords}%
\begin{coordinatesystem}{axis cs}
\PGFPlots\ provides a new coordinate system for use inside of an axis, the ``axis coordinate system'', |axis cs|.

It can be used to draw any \Tikz-graphics at axis coordinates. It is used like
\begin{codeexample}[code only]
\draw 
   (axis cs:18943,2.873391e-05) 
|- (axis cs:47103,8.437499e-06);
\end{codeexample}
\begin{codeexample}[]
\tikzstyle{every pin}=[fill=white,
	draw=black,
	font=\footnotesize]
\begin{tikzpicture}
	\begin{loglogaxis}[
		xlabel={\textsc{Dof}},
		ylabel={$L_2$ Error}]

	\addplot coordinates {
		(11,     6.887e-02)
		(71,     3.177e-02)
		(351,    1.341e-02)
		(1471,   5.334e-03)
		(5503,   2.027e-03)
		(18943,  7.415e-04)
		(61183,  2.628e-04)
		(187903, 9.063e-05)
		(553983, 3.053e-05)
	};

	\node[coordinate,pin=above:{Bad!}] 
		at (axis cs:5503,2.027e-03) {};
	\node[coordinate,pin=left:{Good!}] 
		at (axis cs:187903,9.063e-05)	{};
	\end{loglogaxis}
\end{tikzpicture}
\end{codeexample}

\begin{codeexample}[]
\begin{tikzpicture}
\begin{loglogaxis}[
	xlabel=\textsc{Dof},
	ylabel=$L_2$ Error
]
\draw 
		(axis cs:1793,4.442e-05)
	|-  (axis cs:4097,1.207e-05)
	node[near start,left] 
	{$\frac{dy}{dx} = -1.58$};

\addplot coordinates {
	(5,    8.312e-02)
	(17,   2.547e-02)
	(49,   7.407e-03)
	(129,  2.102e-03)
	(321,  5.874e-04)
	(769,  1.623e-04)
	(1793, 4.442e-05)
	(4097, 1.207e-05)
	(9217, 3.261e-06)
};
\end{loglogaxis}
\end{tikzpicture}
\end{codeexample}

\paragraph{Attention:} Whenever you draw additional graphics, consider using |axis cs|! It applies any logarithms, data scaling transformations or whatever \PGFPlots\ usually does!

There is also a low--level interface to access the transformations and coordinates, see Section~\ref{sec:pgfplots:lowlevel} on page~\pageref{sec:pgfplots:lowlevel}.
\end{coordinatesystem}

\begin{coordinatesystem}{rel axis cs}
The ``relative axis coordinate system'', |rel axis cs|, uses the complete axis vectors as units. That means `$x=0$' denotes the point on the lower $x$ axis range and `$x=1$' the point on the upper $x$ axis range (see the remark below for |x dir=reverse|).

\pgfplotsexpensiveexample
\begin{codeexample}[]
\begin{tikzpicture}
\begin{axis}

	\addplot3[surf] {x^2 - y^2};
	\draw  (rel axis cs:0,0,1) 
		-- (rel axis cs:1,1,1);
\end{axis}
\end{tikzpicture}
\end{codeexample}

\pgfplotsexpensiveexample
\begin{codeexample}[]
\begin{tikzpicture}
\begin{axis}[
	xlabel=$x$,
	ylabel=$y$,
	zlabel=$z$,
	every axis x label/.style={
		at={(rel axis cs:0.5,-0.15,-0.15)}},
	every axis y label/.style={
		at={(rel axis cs:1.15,0.5,-0.15)}},
	every axis z label/.style={
		at={(rel axis cs:-0.15,-0.15,0.5)}},
]

	\addplot3[surf] {x*(1-x)*y};
\end{axis}
\end{tikzpicture}
\end{codeexample}

	Points identified by |rel axis cs| use the syntax

		|(rel axis cs:|\meta{x}|,|\meta{y}|)| or

		|(rel axis cs:|\meta{x}|,|\meta{y}|,|\meta{z}|)| 
	
	\noindent where \meta{x}, \meta{y} and \meta{z} are coordinates or constant mathematical expressions. The second syntax is only available in three dimensional axes.

	There is one specialty: if you reverse an axis (with |x dir=reverse|), points provided by |rel axis cs| will be \emph{unaffected} by the axis reversal. This is intended to provide consistent placement even for reversed axes. Use |allow reversal of rel axis cs=false| to disable this feature.

There is also a low--level interface to access the transformations and coordinates, see Section~\ref{sec:pgfplots:lowlevel} on page~\pageref{sec:pgfplots:lowlevel}.
\end{coordinatesystem}

\begin{predefinednode}{current plot begin}
	This coordinate will be defined for every plot and can be used is \meta{trailing path commands} or after a plot. It is the first coordinate of the current plot.	
\end{predefinednode}

\begin{predefinednode}{current plot end}
	This coordinate will be defined for every plot. It is the last coordinate of the current plot.	
\end{predefinednode}

\begin{pgfplotskey}{allow reversal of rel axis cs=\mchoice{true,false} (initially true)}
	A fine-tuning key which specifies how to deal with |x dir=reverse| and |rel axis cs| and |ticklabel cs|.

	The initial configuration |true| means that points placed with |rel axis cs| and/or |ticklabel cs| will be at the same position inside of the axes even if its ordering has been reversed. The choice |false| will disable the special treatment of |x dir=reverse|.
\end{pgfplotskey}
