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I don't know of a way to do this with the cursor but it could be done by interpolation using the source data. I'm assuming that the graph is in Excel, generated by data in Excel. The accuracy of the interpolation would depend on how close together the data points are, but just by eyeballing it, it looks like linear interpolation would be a good approach here.

You can get fancier by using macros to do the interpolation based on some Y value you select, so it could be interactive.

I could provide a more concrete answer if you can attach your file. Under the text box where you type your reply click the Go Advanced button. On the next screen scroll down and click on Manage Attachments, which will show a pop-up window to Select and Upload a file. Then close the window.