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You might try these demos:

Graphical editor and bunch of other graphical stuff: http://john.virtuallypreinstalled.com/ToolkitDraw

iTunes demo made with Vaadin http://demo.vaadin.com/VaadinTunesLayout/

According to this example: http://demo.vaadin.com/Coverflow/ It seems like you can add custom widgets easily. In this app they started using a Flash Widget for custom graphics.

I'm not a GWT developer, does anyone have any experience adding custom GWT/Vaadin widgets to your applications?

Book of Vaadin http://vaadin.com/book has a whole chapter on this. Also the Vaadin Eclipse Plugin makes getting started with adding new widgets trivial (just "Add New Widget" and it creates a widget stub for you)

Because of the server-driven architecture, you can use graphing libraries like JFreeChart easily.