Drawing over other widgets/windows

["Andrew \"Silver Blade\" Greenwood" <lists silverblade co uk>]

Hi everyone, I'm new to GTK so please bear with me if I'm asking 
something obvious!

I'm in need of a toolkit which will allow me to draw some images over 
existing widgets. This is in order to re-create a GUI simiar to an 
existing prorgam called Reason, where you link up audio modules via the 
use of cables - these cables are drawn over the device modules and some 
controls.

I've yet to find anything that'll let me do this, and am reluctant to 
make my own solution.

How easy is it to do this, if it is indeed possible at all?