Chart Point Coordinates from Plot Picture

I have some plot pictures (i.e. PNG files) for which I want to generate XY points that I can use to create XY scatter charts and, ultimately, trendline formulae. Once I get the XY point coordinates I can do the rest, including converting the Excel point coordinates to plot coordinates (mine are decibels vs. decibels). I fooled around with dropping curve points on the plot pictures and it works great, but do I really need to use VB to get the point coordinates? It would be great if there was a way to right-click something and get the point coordinates, even if I had to do it for every point individually. If I need VB, though, I could use a hint of what function to use. Thanks!

Here's the Excel file showing where I'm stopped. The stuff below the plot image was my crude attempt to approximate the point coordinates by eyeball.

Well, "physician, heal thyself"? It's crude, but using the image control feature described in that "677831-find-coordinates..." thread I managed to get something satisfactory, if not as automated as I'd like. I'm attaching my (now macro enabled) file that shows the point coordinates (Y-Y) I obtained, the mapping from chart coordinates to Pout vs. Pin (dBm-dBm) plot coordinates, then to Gain vs. Pin (dB-dBm) plot coordinates, and the chart of Gain vs. Pin plot coordinates with two polynomial trendlines: a 4th order one for the first five data points (green) and a 6th order one for the last 13 data points (blue) where the 5th data point is used for both trendlines. I'll switch between the two trendline formulae at their Pin = -3.5 dBm crossing. If anyone has a more elegant way of getting chart coordinates from plot images then I'd love to hear it!