Rotor Tip Speed vs Forward Airspeed - IAS or TAS?
For this graph, only TAS can be correct.

More precisely, you should have the same kind of speed on both axes. Otherwise, you would need to qualify the altitude.

Your tip speed (the vertical axis) is (presumably) the simple geometric speed, which depends only on RPM. Or in other words, TAS (with no wind). It will have simple and universal linear relationship with forward TAS. But with IAS, it will be valid only at a certain altitude.

This could be both IAS, but I don't think helicopters just increase their rotor RPM with altitude the way airplanes increase TAS. This could be true for very slow rotors, but the real ones seem to be Mach-limited, so IAS is just not very useful. But correct me if I'm wrong.