Hello Stefan, the problem has nothing to do with the groupplots. The error comes from the "outside legend". Here my mimimal example showing the problem:
% ----- minimal example -----
\documentclass{minimal}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepgfplotslibrary{external}
\tikzexternalize
\begin{document}
\begin{tikzpicture}
\begin{axis}[
legend entries = a,
legend to name = named,
]
\addplot coordinates { (0,0) (1,1) };
\end{axis}
\end{tikzpicture} \\
\tikzexternaldisable % <-- comment me to show the error
\ref{named}
\tikzexternalenable
\end{document}
% ---------------------------
So to circumvent the problem for now you could just exclude the "outside
legends" from externalization as shown above until the problem is fixed.
Best regards
Stefan
> -----Original Message-----
> From: Stefan Schulze Frielinghaus [mailto:ste...@seekline.net]
> Sent: Sunday, October 10, 2010 10:16 PM
> To: pgfplots-features@lists.sourceforge.net
> Subject: [Pgfplots-features] groupplot, legend outside of an axis and
> externalizing everything
>
> Hi all,
>
> I'm using groupplot because it is really easy to draw things like an arrow
> between two plots. However, I also need to use the external library. Now I
> tried to create a legend outside of an axis but this does not work as long
> as I'm using the external library. Here is a minimal
> example:
>
> \documentclass{minimal}
> \usepackage{pgfplots}
>
> \usetikzlibrary{pgfplots.groupplots}
>
> \pgfplotsset{compat=1.3}
> %\usepgfplotslibrary{external}
> %\tikzexternalize
>
> \begin{document}
> \centering
> \begin{tikzpicture}
> \begin{groupplot}[group style={group size=2 by 1, horizontal sep=2cm}]
> \nextgroupplot[legend entries={a;,b},
> legend to name=named,
> ]
> \addplot {x};
> \addplot {x*x};
> \nextgroupplot[xmin=1, xmax=2]
> \addplot {x};
> \addplot {x*x};
> \end{groupplot}
> \draw[thick,black,->,shorten >=20pt,shorten <=2pt]
> (group c1r1.east) -- (group c2r1.west); \end{tikzpicture} \\
> \ref{named} \end{document}
>
> If you uncomment the two lines then you will get an compile error. Does
> someone has an idea how to circumvent this?
>
> Regards,
> Stefan
>
>
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