[linux-audio-dev] [OT] 2-D slider widget?

Has anyone written code for, or even just seen, a 2-D slider widget? What toolkit was it in? Do people around here prefer QT to GTK+, or vice versa, and does anyone use FLTK, or something else? I think the ideal widget would allow the user to click or click-and-drag a cursor around the 2-D area, returning x and y coordinates of course; and would allow a bitmap to be displayed underneath the cursor; and would allow some way to constrain the cursor movements (eg within a circular area inside the widget). Such a thing would be great for lots of audio applications, like controlling filter cutoff and resonance at the same time. That's what I'll be doing, only a bit more complicated..!