You could try (untested)

\def\googlechart[#1][#2]% {\externalfigure[#1][...]% \endgroup}

Thanks, but it has the same result.

\def\dogooglechart[#1][#2]% {\useexternalfigure[googlechart][#1]% \endgroup \externalfigure[googlechart][...]}

That screws up the url in the css (gives a url(googlechart) in the css). Btw. this is not really about google charts, I just took that as an example. Adam