I am not able to reproduce your issue. I used the following as data,
The s1p file
! symbol freq-unit parameter-type data-format keyword impedance-ohms
# MHZ S MA R 50
! freq magS11 angS11 (commented header line)
4.000 0.892 -12.135
5.000 0.891 -18.179
6.000 0.890 -24.193
The MATLAB Script,
hs = sparameters('s1pfile.s1p');
l = rfplot(hs)
l.LineWidth = 5;
Do you face any error while plot ? If yes, can you post the complete error?