Nothing fancy about carpet plots other than that they look fancy. Just a drafting sleight of hand to get multiple data sets onto the one bit of paper. Confused me for an hour or so when I first had to work out how it was done (50-odd years ago, now). Once it fell into place, it's a dead easy technique to apply. My approach when putting line segments into the PC is to run regressions on each of the segments and then play with the results.
Main thing is to break the segments up (as necessary) into smaller lengths so that they can be fitted with no more than a cubic. Higher orders can give you headaches with unwanted inflexions .. especially if you want to play with short extrapolations. One has to keep in mind the goal is that we are trying to replicate the AFM pictures, not the physics behind the lines.
My comment about Excel, elsewhere, was not to suggest you use it to run the regressions. However, it is a fine vehicle to structure the final data output engine. I don't even bother using Fortran any more in preference .. does anyone remember Fortran ?