You will sooner or later need to quickly align a series of shapes. OmniGraffle has several ways of doing this.
Start by placing a few shapes randomly on the canvas.
From the Arrange | Align menu, you have several choices on how you want to align your shapes. It is also possible to right-click and get a context sensitive menu appearing with the various alignment choices.
Most of the same choices are also available from the Canvas: Alignment inspector.
However, there are a few extra choices available like offsetting shapes ...