If you need to work on more than one diagram for your task, you have a few choices: You can either have one diagram per OmniGraffle file, you can place one diagram on each page on your OmniGraffle canvas, or you could use more than one canvas.
A lot of people never get past the idea of working with more than one canvas. They are more than happy using one page for each diagram that they create and exporting or printing just the very page that they need.
You should aim higher and become confident with the notion of one diagram per canvas. This chapter will teach you exactly how to work with multiple canvases.
Another wonderful, time saving, and efficiency booster is the use of canvas layers, or just layers for ...