If either the X or Z axis Factors is set to 0, then no interpolation is done and no range can be selected on that axis.
Program creates a new multifile with the larger number of points and subfiles.
This uses a modified 'spline-like' slope matching algorithm. A different cubic polynomial is derived for each 4 point region. Y1 and Y2 are the Y values of these two end point points. Also,Y0 and Y3 are the Y values of the points just before and after the former. It is assumed that the slope at a given point is the average of the slope between it and the previous point and the slope between the next point and itself.