The graphviz.pm plug-in currently attempts to read PNG data in UTF-8 mode, which sometimes fail with a message similar to the following (depending on the input):
utf8 "\x89" does not map to Unicode at /usr/local/lib/perl5/site_perl/5.8.8/IkiWiki/Plugin/graphviz.pm line 53, <IN> chunk 1.
Wide character in subroutine entry at /usr/local/lib/perl5/site_perl/5.8.8/IkiWiki/Plugin/graphviz.pm line 68.
It also generates image URLs relative to the page being rendered, which means the URLs wont work when previewing a graph from the CGI script.