Custom drawing on a TChart descendent

We had developed a descendent of the TChart component where we did our own
drawing in the overriden public procedure Draw(UserCanvas:TCanvas; Const
UserRect:TRect). However, we have discovered that in later releases of
TeeChart it does not seem to respond to WM_PAINT messages, ie, it draws fine
but if you move a window across the chart it does not draw the parts we have
added (but draws everything else).
Maybe it never did and we just didn't notice! Anyway, any suggestions on how
to fix this would be appreciated. We use Delphi 5 Enterprise and TeeChart
Pro VCL version 4.