REad about hold on. After plotting the first, use hold on. Alkso you may go ahead like below:
p = polyfit(wspd_3yr,msid_3yr,1)
yi = polyval(p,wspd_3yr,msid_3yr) ;
Also when you use:
mdl = fitlm(wspd_3yr,msid_3yr,'linear') ;
mdl has coefficients of the line, you can use polval like shown above to plot.