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? 


