Hi,
You can plot the real part of s parameters by setting the 'plotflag' as 'real'.
As an example , if 'hs' is your S-parameters object, to plot the real part of 'S21' you can use the following line of code
rfplot(hs,2,1,'real')
Similarily, you can also plot the imaginary value, absolute value and angle of the S parameter. You may refer to the following documentation to get a better idea on it.
https://www.mathworks.com/help/rf/ref/rfplot.html