This is the same bug as 2276946. Sorry for duplicate but I don't know how attach files to an open bug.
Epydoc from svn don't create any graph (debian epydoc-3.0.1 does) nor show any warning or error.
epydoc svnversion 1812 as of 20090806
$ dot -V
dot - Graphviz version 2.20.2 (Mon Jun 29 17:34:52 UTC 2009)