Generate the plot and modify the axis values. HAXIS= sets major tick marks for the horizontal axis. VAXIS= sets major tick marks for the vertical axis. HMINOR= and VMINOR= specify the number of tick marks between major tick marks. 
proc gplot data=reflib.stocks;
   plot high*year / haxis=1955 to 1995 by 5
                    vaxis=0 to 6000 by 1000
                    hminor=3
                    vminor=1