externalizing groupplots [closed]

Question

I'm using the pgfplots-package. Being very new to it I just found out how to use the externalize command. Now after successfully compiling my worksheet I looked up in the progam directory finding more externalized plots than there are in my document.

This "problem" especially occurs with the groupplot command. That means a plot with three groups is externalized three times. Besides there are double question marks on top of the groupplots. Does somebody now why this happens? And how I can solve this problem?

I'm using MikTeX 2.9 and compiled the document with pdflatex -enable-write18 <file name>.

MWE:

\documentclass{article}
\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{pgfplots.groupplots}
\usetikzlibrary{external}
\tikzexternalize[shell escape=-enable-write18]
\pgfplotsset{compat=newest}


\begin{document}

\begin{tikzpicture}
\pgfplotsset{every y tick scale label/.style={at={(0,-\pgfplotsvalueoflargesttickdimen y -0.3em)},anchor=east,inner sep=0pt}}
    \begin{groupplot}[  
    group style= {  columns=3,xlabels at=edge bottom,
                y descriptions at=edge left,
                horizontal sep=0cm,group name=plots},
    width=0.4\textwidth,
    height=0.45\textwidth,
    ymin=0, ymax=40,
    enlarge x limits={rel=.5},
    ybar,ylabel={ylabel},
    /pgf/bar shift=0pt,/pgf/bar width=0.5cm,
    xtick=data,legend columns=3,
    xtick pos=left,
    ymajorgrids=true,]
    %  \Rapperswil
    \nextgroupplot[xlabel=Rapperswil]
    \addplot coordinates {(1,10)(2,15)(3,20)};
    % \Palermo
    \nextgroupplot[xlabel=Palermo]
    \addplot coordinates {(1,20)(2,22.5)(3,25)};
    % \Jeddah
    \nextgroupplot[xlabel=Jeddah]
    \addplot coordinates {(1,30)(2,5)(3,17)};
  \end{groupplot}

\end{tikzpicture}

\end{document}