Generate two plots and display them on the same set of axes. OVERLAY specifies that both plot lines appear on the same graph. LEGEND= assigns the LEGEND1 definition to the graph.
proc gplot data=reflib.stocks;
   plot high*year low*year / overlay legend=legend1
                             vref=1000 to 5000 by 1000 lvref=2
                             haxis=axis1 hminor=4
                             vaxis=axis2 vminor=1;