Hi Bruce,

My idea is to somehow define a "mouse down" macro in the graph. The macro
would need to get the Col 1 value of the point, turn that into a row in
the worksheet and set the Col 3 value in that row to 0. I can handle the
"find the right row and set col 3 to 0" part assuming I have the col 1
value, but I'm not sure how to handle the "I clicked on a point in a graph
and here are the coordinates" part of the problem, or even if it's
possible,

Jon Peltier does a great job of explaining this at:

http://www.computorcompanion.com/LPMArticle.asp?ID=221

Regards

Stephen Bullen
Microsoft MVP - Excel

Professional Excel Development
The most advanced Excel VBA book available
www.oaltd.co.uk/ProExcelDev