For plotting,
IDL> mfreq=0 <CR>
IDL> norp_plot,mfreq,file,mvalid,time,fi <CR>
where mfreq is the index number of the plotting frequency
in the array freq.
To overlay another plot
IDL> mfreq1=0 <CR>
IDL> norp_plot,mfreq1,file,mvalid,time,fi,/over <CR>
Or by directly;
IDL> utplot,time(where(mvalid(mfreq,*))),fi(mfreq,where(mvalid(mfreq,*))) <CR>