Please help me construct a graph...

I have no experience with this sort of problem.

I've only ever made simple graphs before. This one requires special computer software.

For now, the problem is simple...

I have derived a formula for a simple model of a star:

$\displaystyle P=P_0 (r/r_0)^a$ ; $\displaystyle T=T_0 (r/r_0)^b$

I need to make a graph of a vs b.

This is where I'm begging you to help me. What (free) program could I use? Could anyone provide instructions on how to graph this with the program?

I thank you very very much in advance!