Pete,
You might like to look at a demo program I made as part of my GTK+
education - it's a functional xy-controller:
http://www.skynet.ie/~jmmcd/software/xy-controller.tar.gz
But I actually made the xy-controller directly from the drawingArea
widget. Ie, I didn't make a new reusable widget (maybe it'll inspire
you to do that for PHAT), because that seems like a lot more work!
Maybe when I learn a bit more...
James