This is a windows notification code that is sent to the parent window through the WM_COMMAND message when the mouse moves over the extended plot area.

See CChartViewer.setPlotAreaMouseMargin on what is the extended plot area.

When handling this event, CChartViewer.getChartMouseX, CChartViewer.getChartMouseY, CChartViewer.getPlotAreaMouseX and CChartViewer.getChartMouseY can be used to obtain the mouse cursor position in the chart pixel coordinate system.