http://plan9.bell-labs.com/plan9/addons.html has incorrect (presumably outdated) info on the Graphviz port. The graphviz folks themselves seem to no longer have the 1.10 source distribution available for whatever reason. No loss, though: the supposed patch link has been replaced. It now redirects to a page with an updated download which has the patched version all packaged up. Instructions are on the site, but it's the standard 'mk install' process. I've confirmed this package works as described.
Oh, and it's graphviz, not graphvis
