ITChart.ChartWidth
ITChart

property ChartWidth: Integer;

Type Library
TeeChartx

Description
Run-time and read only. The ChartWidth property indicates the total width in pixels between the Left and Right Chart Axis positions.

It is equal to:

ChartWidth=TChart1.GetChartRect.Right - TChart1.GetChartRect.Left

This IS NOT the same as Chart1.Width, because Chart margins, Axis margins, Legend width, etc are first substracted from the Chart horizontal size.