Fill a quadrant with PGFplots and axis equal=true

Question

I need to fill quadrant three and want to use axis equal=true. However, using this setting changes the minimum value of x. How do I adjust this to fill the entire quadrant independent of axis equal=true or axis equal=false.

The MWE below produces the result I want, but not if I uncomment the axis equal=true line.

\documentclass{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}

\newcommand*{\XAxisMin}{-4.5}
\newcommand*{\XAxisMax}{4.5}
\newcommand*{\YAxisMin}{-4.5}
\newcommand*{\YAxisMax}{4.5}

\begin{axis}[%axis equal=true,
    axis y line=center, axis x line=middle, axis on top=true,
    xmin=\XAxisMin, xmax=\XAxisMax, ymin=\YAxisMin, ymax=\YAxisMax, 
] 

\fill[red!40]
    (axis cs:\XAxisMin,\YAxisMin) -- 
    (axis cs:0.0,\YAxisMin) -- 
    (axis cs:0.0,0.0) -- 
    (axis cs:\XAxisMin,0.0) -- 
    cycle;

% Based on Jake's solution, this should have worked, but produces identical results for me (PGFversion: 2.10)
%        \fill[green!40]
%       (axis cs:\pgfkeysvalueof{/pgfplots/xmin},\pgfkeysvalueof{/pgfplots/ymin}) -- 
%       (axis cs:0.0,\pgfkeysvalueof{/pgfplots/ymin}) -- 
%       (axis cs:0.0,0.0) -- 
%       (axis cs:\pgfkeysvalueof{/pgfplots/xmin},0.0) -- 
%       cycle;


\end{axis} 
\end{tikzpicture} 
\end{document}

Answer 1

Threeapproaches:

• Use the rel axis cs coordinate system, which places (0,0) in the lower left corner of the plot and (1,1) in the upper right
• Use the anchors current axis.left of origin and current axis.below origin, which are the projections of the axis origin on the left and lower plot edge, respectively
• or, if you're using the CVS version of PGFplots read the current value of xmin and xmax using \pgfkeysvalueof{/pgfplots/xmin}

Example:

\documentclass{article}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}

\newcommand*{\XAxisMin}{-4.5}
\newcommand*{\XAxisMax}{4.5}
\newcommand*{\YAxisMin}{-4.5}
\newcommand*{\YAxisMax}{4.5}

\begin{axis}[axis equal=true,
    axis y line=center, axis x line=middle, axis on top=true,
    xmin=\XAxisMin, xmax=\XAxisMax, ymin=\YAxisMin, ymax=\YAxisMax, 
] 

\fill[orange!50] (axis cs:0,0) rectangle (rel axis cs:1,1);

\fill[green!70!black] (current axis.right of origin) rectangle (current axis.below origin);

\fill[blue!50]
    (axis cs:\pgfkeysvalueof{/pgfplots/xmin},\pgfkeysvalueof{/pgfplots/ymax})
    rectangle (axis cs:0,0) node [pos=0.5,text=white] {CVS only};

\end{axis} 
\end{tikzpicture} 
\end{document}