If you are willing to use a regression trendline, then contrary to what the
documentation might lead you to believe, you are not restricted to a linear
equation. For more see the 'Regression with polynomials' section in
Trendline Coefficients and Regression Analysis
http://www.tushar-mehta.com/excel/ti...efficients.htm

If you want to use a piecewise linear interpolation see
Interactive Chart
http://www.tushar-
mehta.com/excel/software/interactive_chart_display/index.html

and

Interpolation
http://www.tushar-mehta.com/excel/ne...ion/index.html

--
Regards,

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

In article ,
says...
Is it possible to get exact values from a graph that isn't one of your data
points? When I graph nonlinear variables is there some type of formula I can
type in to get me an exact x value for a specific y value. For example i have
x values of 0, 1, 2.5, 5, 10.... and y values of 100, 79, 55, 32, 12. If I
want to know what x equals when y=50 how do i go about doing that? I know
that I can't use the standard y=mx+b because my data is not linear... any
suggestions?? I've been trying to figure it out for a couple of weeks and its
starting to drive me crazy. Ive looked into a couple of other graphing
programs (Igor Pro, GraphPad Prism and Delta Graph 3 and they have been no
help either!)

Thank you!