Download the freeware
Shape On You and use it to create the view you want. Insert the image into OmniGraffle - if you need to edit the shape, it utilises LinkBack (as does OmniGraffle) so double clicking the inserted shape will cause ShapeOnYou to re-open the image. Any changes you make will also occur back in the OmniGraffle document when you save the edits in ShapeOnYou.