does omni graffle have the concept of containers like Visio?
 

Visio's containers explained here:

[will post the link once the forum lets me]

[URL="http://office.microsoft.com/en-us/visio-help/clarify-the-structure-of-diagrams-by-using-containers-and-lists-HA010369718.aspx?CTT=1"]Info about Visio's container feature[/URL]

I don't know much about Visio containers other than the link provided, but it seems that there are two OG features that relate.

Simple groups have the same basic behavior as containter as described in the link's page - moving, copying, deleting as a group. Manipulation and addition methods are simpler, though.

Subgraphs in OG put a visible border around grouped objects, have a title, and can be expanded and collapsed. You can control their style features. While subgraphs apply primarily to elements of outline diagrams, any set of objects can be grouped as a subgraph.