[cairo] erasing overlay on GTK drawing area widget.

[Michael K]

I'm attempting to have two GtkDrawingArea widgets in a GtkOverlay container.
The lower widget is a graph and the top widget shows dynamically changing information related to the graph based on the mouse position.
I cannot seem to erase the top drawing area without wiping out the graph on the bottom drawing area.
Is this at all possible or am I just doing something wrong ?
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