Interpolate only one axis

What is the easiest way to interpolate only the Y coordinates?

I want to adjust the vertical contrast of the regular master. The easiest way would be to interpolate the Y coordinates with the bold master. Since the transformation filter “background” does not support that option I did the example below half-manually with a UFO file. I always wondered why RMX Tuner doesn’t make use of that simple trick. Am I wrong or isn’t that process essential to all typedesigners? Wouldn’t it make sense to implement it to with a little axis drop down menu in the transformation window?

I would use an InterpolationWeightY parameter in the instance.

Thanks! I guess it’s time I dive more into that stuff. There is more possible than I already know.