Version 1.8.9 of media-gfx/graphviz has been released some time ago, here's an ebuild for it.
Created attachment 3958 [details] graphviz-1.8.9.ebuild Here's the ebuild.
Since this has been reopened, you may as well note that version 1.8.10 has been released in the meantime. Simply changing the name of the ebuild file to use the newer version works perfectly, so I'd recommend doing that instead of leaving it at 1.8.9.
*** Bug 8408 has been marked as a duplicate of this bug. ***
already 1.8.10 in portage tree