BCGControlBar Pro for MFC
UINT BCGM_ON_CHART_AFTER_RECALC_LAYOUT

The framework sends the BCGM_ON_CHART_AFTER_RECALC_LAYOUT when the chart has recalculated its layout.

Parameters
wParamID of chart visual object.
lParamNone.
Returns
the return value is ignored.

You can process this message to implement interactive charts. You might need to invalidate some cached data when the layout has been recalculated.