first, you need to calculate the ECC following these steps to plot ECC vs Freq plot in HFSS calculated from S parameters:
In the project manager window right click on results and then select 'Output Variables'.
Provide Name to the parameter and then insert the expression: (mag(conjg(S(1,1))*S(1,2)+conjg(S(2,1))*S(2,2)))^2/((1-mag(S(1,1))^2-mag(S(2,1))^2)*(1-mag(S(2,2))^2-mag(S(1,2))^2))
Click on Add and then click on Done.
Then again right click on results --> Create Modal Solution Data Report --> Rectangular Plot --> Under Category option select 'Output Variables' --> Select Name of the parameter that you have given in 2nd step.