If you can accept piece-wise linear interpolation, see
Interactive Chart
http://www.tushar-
mehta.com/excel/software/interactive_chart_display/index.html

--
Regards,

Tushar Mehta
www.tushar-mehta.com
Excel, PowerPoint, and VBA add-ins, tutorials
Custom MS Office productivity solutions

In article ,
says...
I made a line graph of data to use as a calibration. I know the y value and I
want to find the X value. Is there a way that I find find this specifically
on the line without using a trendline formula or guessing by looking at the
gridlines?