Attached is a vi that extracts the points in the displayed area of a waveform graph.
The basic idea is first to find the array subset corresponding to the x scale, after readind the xscale max and min. Then to find in this array subset the left and right points that are in the range defined by the yscale min and max. Since a waveform graph is a 1D array of evenly spaced data, this is the simplest way of extracting a 1D set of data. Of course, if some data between the left and right hand sides were out of the Y range, they would be included in the extracted set. There is no way to exclude such points without building also a corresponding X array.
The adaptation to XY graphs should not be much more complicated.
Hope that this is what you were expecting.
CC
Chilly Charly (aka CC)