Have you tried Umlet http://www.umlet.com/ ?
I suspect its more of a problem with the fundamental design of UML though. I think if there was like, a UML-lite spec or something. To me, i feel like UML is to what I want to use as XML is to JSON (or csv for that matter), if that makes any sense.
Can I also suggest Kite9?
www.kite9.com/kite9/gui.html
Which does automatic diagram layout. It's not quite doing UML yet, but a reasonable approximation thereof. It's more visual/drag-and-drop than UMLet.
I intend to include a suite of actual UML symbols within the next week or so.
Hopefully this will be useful to you!
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