Ok, I'm starting to understand a little more. Since you will be selected one point at a time (from each curve) you can use the cursors. Have one cursor snap to Plot 0 and one cursor snap to Plot 1. this will prevent the user from selected two points from the same curve. You can programatically get the values from the cursor position using property nodes (attached image shows how to retrieve XY values of cursor zero).
As you cycle through the test points, populate a third XY pair using response values from test subject. Start with an array of say, -1 for y values restricting the y axis to positive values, this will essentially hide the values which are not populated.
When you have a new data point to enter, scan the X array to find the position of the test frequency and replace the corresponding Y value. (see example image : NON-functioning code to quickly demonstrate a possible option)
