backbone:
If i correct the X for i then after the switch point the line goes straigth roof high. See screenshot.
If you're trying to store a curve in the array, why don't you just calculate the values you want and put them in a static initialiser?
If you're trying to approximate the curve with three straight lines then I don't see the benefit of the lookup table - you might just as well do a range check, scale and offset. If execution time is critical then you could implement that logic during setup and cache the result, but I'd still calculate it doing three linear transforms rather than trying to write it as a single expression. But the circumstances where it's sensible to pre-calculate a linear transform are pretty unusual imo.