Based on what I've read (and this isn't official from Microsoft) Excel uses Bezier interpolation to create smooth curves from three or more points. I played around with polynomials and it never looked right. Increasing the number of controls points generated larger polynomial equations and the curves didn't look like what I want. That's how I learned about Beziers and subsequently splines. I tried both of those modules, and now the Interpolate one. Here is an example of Spline vs. Interpolate. Also in this graph is the Interpolate with three control points. As you can see the three points makes just a parabola (p<0). https://www.dropbox.com/s/9cnl1jp9zzzhh19/Interpolate_example1.1.pdf

So far only Math::Interpolate has given me a shape that is close enough to what I need with 5 points (three real points and two that are guesses). The problem is the height of each point. That has to be right. Hence the need to scale.