Turn those blues to violet using Violet 
Violet was the UML modelling tool I had to use to save me some fun-later-time and make the whole process quick and painless. It also supports exporting to an image format and all in all it is a lovable, simple and just-works tool. But do note that it uses Java to do all that magic.
The other alternative I had was Umbrello which sadly isn’t in a very usable state yet. Perhaps this summer its GSoC fix may make it better. Wish I knew enough of QGraphicsView stuff to try it myself 
