Go to "Results". Go to "Antenna Parameters". Select your choice of visualization, I prefer "2D".
Under category select "Realized Gain". From there, you should see at the bottom of the list "RealizedGainCoPolar" and "RealizedGainCrossPolar". You can plot both at once.
You can also plot the delta between the co and cross polarized gain as a function of frequency, by putting this in the 'Y:' part --> "abs(dB(GainCoPolar) - dB(GainCrossPolar))" <-- and selecting "Freq" as your primary sweep instead of theta or phi. You can select theta = main beam direction, and phi = main beam direction, and it'll calculate the delta between your co and cross polarization at that specific direction.