Included in the latest version of OmniGraffle, v7.3, is the ability to create and present your objects, charts, and canvases using Artboard Layers. Previously, Presentation Mode was limited to Canvases; with Artboards, OmniGraffle will zoom in on to the exact border of your Artboard which means a lot more flexibility.
And because Artboards can be used to slice and dice your Canvas however you want, they can be used to present the same object a billion different ways. Check this out:
Also included: panning, zooming, and a sidebar that lists both canvases and artboards.