for the graphs to work dev-perl/GD needs to be built with USE=png, otherwise you'll get errors in your Apache logs about methods not existing
Hmmm... That test would require dev-perl/GD atom in DEPEND because, hypotetically speaking, RDEPEND atoms can be installed after the package in question.
I think a user has a reasonable expectation to have all the features for which he/she has enabled USE flags for to be operational at install time. If there is no USE flag then that feature should be enabled. Yes, hypothetically speaking dev-perl/GD can be installed AFTER dspam-web is installed, however Portage doesn't currently do this and it really should be checked for. I think you would need to put it in DEPEND until USE based depends come along and then you can only have it in RDEPEND.
Fixed without revision bump.
