Modify data directly from the graph with GUI

Hello everyone. The principal goal of my GUI is to plot 3 surfaces which all have the same axis (speed, frequency and dB), but coming from different sources. I have a 4th plot which is the sum of those 3. Up to there: everything works.

What I would like to do is: when the surfaces are created and displayed, being able to drag and hence change a point of one of the surface and see the influence on the 4th plot.

I have tried to look around but could not find solution to my problem. I was thinking of using the data cursor to read the point willing to be changed. Then maybe create a new point and use this last one to replace the one indicated on the little tip box. My problem is that I have no idea what to use to transfer the data from my figure interface to the actual code.

Any one has an idea?