Interpolating Y-values on a Line Chart

You probably could create a name (press Ctrl+F3), and use it as a source for additional series to be plotted on a graph.
See The dynamic formula method part here: http://www.techrepublic.com/blog/mic...arts-in-excel/ to see different task, but using the same mechanism.

If that is not enough, and more step-by step instruction is needed, consider attaching small sample/dummy file and give some suggestions about interpolation - just linear?

PS. I'd personally probably just insert these few extra columns (possibly hidden). It is a kind of "small pain - big gain" solution :-D