That graph is obtained by:
Solve this for v:
http://blog.wolfram.com/data/uploads/2012/10/Speed-Equation31.png
Then divide by the speed of sound at that altitude:
http://blog.wolfram.com/data/uploads/2012/10/Speed-Out5.png
You're not going to be able to find the "correct" equation, but the graph somewhat resembles a (negative) quartic graph so you could stick coordinates on the graph into a system of equations to get coefficients. Maybe.