We've got a feature request on file for this in OmniGraffle, but it's on the "Maybe, possibly, someday" list - we don't have any plans in the foreseeable future, in other words. In the meantime, our recommendation has been to use
OmniPlan for this sort of thing.
Of course, if there are reasons why OmniGraffle would be better for some reason, we'd be happy to file that feedback against the existing OmniGraffle request. Thanks!