Tabular like legend in pgfplots

Question

I use legend style={legend columns=*} to create columns in my pgfplots legends. However, this options only changes the way legend entries are displayed. What I often need is to display various data for each plot, like in a table. Here is an example of the (ugly) work-around I use so far:

\documentclass{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
title={$y=a \cdot x + b$},
xlabel=$x$,
ylabel=$y$,
legend pos=south east,
legend style={legend columns=2},
]

\addlegendimage{empty legend}
\addlegendimage{empty legend}
\addlegendentry{$\quad a \quad$} % \quad are just here to make columns wider... I suppose there's a better way
\addlegendentry{$\quad b \quad$}

\addplot+[domain=-1:2, samples=10] {3*x+1};
\addlegendimage{empty legend}
\addplot+[domain=-1:2, samples=10] {2*x+2};
\addlegendimage{empty legend}
\addplot+[domain=-1:2, samples=10] {1*x+3};
\addlegendimage{empty legend}

\addlegendentry{3}  % first data for the first graph
\addlegendentry{1}  % second data for the first graph
\addlegendentry{2}  % first data for the second graph
\addlegendentry{2}  % you've got it...
\addlegendentry{1}
\addlegendentry{3}

\end{axis}

\end{tikzpicture}
\end{document}

I'd be grateful for any ideas to make the syntax cleaner and, above all, robust!

Answer 1

I think the easiest solution is to create your "legend" yourself with the \matrix command. To get the \addplot styles you just need to \label them accordingly and to recall them, use the \ref feature inside a \matrix cell.

For more details please have a look at the comments in the code.

\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
    % load `matrix' library so we can use the `matrix of nodes' feature
    \usetikzlibrary{
        matrix,
    }
    % use `compat' level 1.3 (or higher) to use the advanced placement features
    % for the axis labels
    \pgfplotsset{
        compat=1.3,
    }
\begin{document}
    \begin{tikzpicture}
        \begin{axis}[
            title={$y=a \cdot x + b$},
            xlabel=$x$,
            ylabel=$y$,
        ]
            \addplot+ [domain=-1:2, samples=10] {3*x+1};
                % add labels to the plots
                \label{plot:line1}
            \addplot+ [domain=-1:2, samples=10] {2*x+2};
                \label{plot:line2}
            \addplot+ [domain=-1:2, samples=10] {1*x+3};
                \label{plot:line3}

            % create a (dummy) coordinate where we want to place the legend
            %
            % (The matrix cannot be placed inside the `axis' environment
            %  directly, because then a catcode error is raised.
            %  I guess that this is caused by the `matrix of nodes' feature)
            \coordinate (legend) at (axis description cs:0.97,0.03);
        \end{axis}

        % create the legend matrix which is placed at the created (dummy) coordinate
        % and recall the plot specification using the `\ref' command
        %
        % adapt the style of that node to your needs
        % (e.g. if you like different spacings between the rows or columns)
        \matrix [
            draw,
            matrix of nodes,
            anchor=south east,
        ] at (legend) {
                             & $a$ & $b$ \\
            \ref{plot:line1} & 3   & 1   \\
            \ref{plot:line2} & 2   & 2   \\
            \ref{plot:line3} & 1   & 3   \\
        };

    \end{tikzpicture}
\end{document}