In ADS simply, after the simulation has finisehd, in the results windows open a rectangular plot, add a trace (doesen't matter which one), then in the rigth windows where the trace you choose is listed double click on it. Then go in "Trace expression" and replace what is written with:
plot_vs(<what you want to plot>, freq*freq)
plot_vs means plot versus. the synopsis is plot_vs(Y,X) that will plot Y with respect to X.
If, for instance, you want to plot the S21, in dB with respect to freq^2, you'll have to write:
plot_vs(dB(S(2,1)), freq*freq)