Why must I read SWR on the Smith chart where X=0 ?

Hi does any body know why I the voltage maximum on the transmission line must occur where the imaginary part of the impedance vanishes R=R_max, X=0.

How can I prove that impedance Z around the constant reflexion circle is at its maximum where the imaginary part vanishes.

Couldn't I have Z=sqrt(R^2 + X^2) > sqrt(R_max^2 + 0^2) somewhere along a constant Gamma-circle?

/StoppTidigare

If X=0 means ,no power distrubute for that.
Avilable power is max
So V*V =P*R