For displaying cvs graphs, viewcvs uses cvsgraph when enabling graphs feature (use_cvsgraph=1). So I think cvsgraph should be in RDEPEND of viewcvs (at least as optional)
Adding a USE-flag for just this seems extranous to me, and people will moan pulling it in unconditionally. (Same reason goes for enscript and other such apps that viewcvs can use once enabled.) As it's simply a two-step process (enabling in viewcvs.conf and emerging cvsgraph), resolving WONTFIX.