can GdkDrawing area be invisible or not opaque

[Damon Register <damon w register lmco com>]

So far I have managed to learn enough with Gtk::DrawingArea and Cairo
to make a custom widget.  I started with a clock example.  I am able
to change the cairo drawing within the on_expose_event of the custom
widget in order to draw what I want (just a filled circle).

The Gtk::DrawingArea is a square with height and width 25.  I use
cairo to draw a filled circle in the middle of that square.  The area
outside the circle but within the DrawingArea is left alone.
There is one part that I haven't figured out yet.  The DrawingArea
covers whatever is beneath it but I don't want that.  I would like
only the filled circle to be seen but I want the rest of the
Drawing area to be invisible so it doesn't cover anything.

I have managed to get through a cairo tutorial and tried a few samples
but I don't find anything that covers this.  Would I be correct in
thinking that this is really a concern of Gtk::DrawingArea and its
behavior?

Damon Register