How to extract S31 from s4p file？

Dear friends, how to extract the data in s4p file and draw differential signal image? Using the rfparam (diffsparams, 1,1) command in the RF tool can directly obtain sdd11, but now I want to directly extract the S parameter of the fre-8 * 4 matrix and carry out sdd11 = 0.5 * (s11-s13-s31 + S33). How to extract a frequency point fre corresponding to an 8 * 4 matrix, such as how to extract S13 and S12 corresponding to frequency f1?