ADS manipulate Trace S-Parameters:

[Andrew77]

Hi,
I would like to plot the difference in GAIN of various path (S21 dB)

How can be done in ADS?
Is there an Equation which plots the difference in dB of two or more traces? For Example here is two traces :

 

[BigBoss]

Define an equation like;

Difference1=RHI_G-S(2,1)
then plot it on a separate diagram.