BCGSuite for MFC
BCGCBPRODLLEXPORT UINT BCGM_ON_CHART_LEGEND_MOUSE_DOWN

The framework sends the BCGM_ON_CHART_LEGEND_MOUSE_DOWN message when the legend visual object receives a mouse button down notification.

Parameters
wParammouse button number (0 - left, 1 - right, 2 - middle).
lParama pointer to a BCGPChartLegendHitInfo structure.
Returns
TRUE - disable further mouse down processing. In this case a legend scrolling (if available) will not occur.

You should process this message to implement interactive chart legend.