Hi Paul,

To extrapolate, right-click the series and select 'Add Trendline'
Use the 'display equation' option and go from there.

Otherwise, use LINEST function in your worksheet.

To interpolate, download the 'Reading Interpolated Values' sample from
http://www.edferrero.com/ExcelCharts...2/Default.aspx

Ed Ferrero
www.edferrero.com