Custom legend order in pgfplots with externalization

Question

I'm struggling with getting a custom legend order in my pgfplots groupplot. I tried this (esdd's answer) but I get the error Undefined control sequence. {\patchFailedError}

Stefan Pinnow says one answer later, that it should work the following way:

\documentclass{article}
\usepackage{pgfplots}
\usetikzlibrary{
    matrix,
    pgfplots.external,
    pgfplots.groupplots,
}
\pgfplotsset{compat=1.3}
\tikzexternalize
\begin{document}

\begin{figure}
    \tikzset{external/force remake} 
    \tikzsetnextfilename{ext}
    \begin{tikzpicture}
        \begin{groupplot}[
            group style={
                group size= 2 by 1,
                xlabels at=edge bottom,
                ylabels at=edge left,
                x descriptions at=edge bottom,
            },
            ]
            \nextgroupplot[]
            \addplot [color=black] coordinates {(0,0) (1,0)};
            \addplot [color=red] coordinates {(0,1) (1,1)};
        \nextgroupplot[name=aa]
        \addplot [color=black] coordinates {(0,0) (1,0)}; \label{leg:a}
        \addplot [color=red] coordinates {(0,1) (1,1)}; \label{leg:b}

    \end{groupplot}


\matrix [
matrix of nodes,
nodes={anchor=west},
anchor=north east,
at={([shift={(-3pt,-3pt)}]aa.north east)},
fill=white,
draw,
inner sep=2pt,
row sep=2pt
] {
    \ref{leg:b} $b$ \
    \ref{leg:a} $a$ \
};
\end{tikzpicture}

\end{figure}
\end{document}

But I get this output:

Hence, the plot marks are missing. How can I achieve a custom legend order in my case? Disable externalization is no option (also not for this one plot).

Answer 1

you have to define the position outside the node environment and then it works like expected.

\documentclass{article}
\usepackage{pgfplots}
\usetikzlibrary{matrix}
\usepgfplotslibrary{groupplots,external}
\pgfplotsset{compat=newest}
\tikzexternalize
\begin{document}

    \begin{figure}
        \tikzset{external/force remake} 
        \tikzsetnextfilename{ext}
        \begin{tikzpicture}
            \begin{groupplot}[
                group style={
                    group size= 2 by 1,
                    xlabels at=edge bottom,
                    ylabels at=edge left,
                    x descriptions at=edge bottom,
                },
                ]
                \nextgroupplot[]
                \addplot [color=black] coordinates {(0,0) (1,0)};
                \addplot [color=red] coordinates {(0,1) (1,1)};
            \nextgroupplot[name=aa]
            \addplot [color=black] coordinates {(0,0) (1,0)}; \label{leg:a}
            \addplot [color=red] coordinates {(0,1) (1,1)}; \label{leg:b}

        \end{groupplot}

        \matrix [
        matrix of nodes,
        anchor=north east,
        fill=white,
        draw,
        inner sep=2pt,
        row sep=2pt
        ] at {([shift={(-3pt,-3pt)}]aa.north east)} {
            \ref{leg:b} & $b$ \\
            \ref{leg:a} & $a$ \\
        };

    \end{tikzpicture}
\end{figure}


\end{document}