I have the following function, depicted on this image:

The strange thing is, that I only know the coordinate system from the "inside out", that means I know the lengths b, c, e and f, but I don't know a and d. Neither do I know the function. My goal is to calculate a and d. I could assume the function is quadratic, (even if it's not, it would be sufficient as an approximation, if I swapped the two axes).
I also know the length a+d. It's as if the two axes were a thread being stretched over the edge of a cube, and I can move it, and I know all the markings on it, but not the position where it contacts the edge.
The problem is, if I try to use my assumptions on the function for my calculations, I end up with
g(a) = d
g(a+b) = d + e
g(a+b+c) = d + e + f
As I don't know a and d, this results in a big mess in my calculations, and I can't get anywhere, because I don't know the origin of my coordinate system.
As I know the "outside" part of my coordinate system better than the "inside" of it, is there any transformation I can use to make my job easier? (or a completely different strategy to solve my original problem?)