Generate the plot and modify the axis values. The VAXIS= option sets major tick marks for the vertical axis. The HMINOR= and VMINOR= options specify the number of tick marks between major tick marks.

proc gplot data=stocks;
   plot high*year / haxis=1955 to 1995 by 5
                    vaxis=0 to 6000 by 1000
                    hminor=3
                    vminor=1