Generate a plot with filled areas. The plot requests are ordered to draw the lowest plot first. Area 1 occupies the space between the lowest (first) plot line and the horizontal axis, and area 2 is below the highest (second) plot line. This arrangement prevents the pattern for area 1 from overlaying the pattern for area 2. AREAS=2 fills all the areas below the second plot line. 
proc gplot data=reflib.stocks;
   plot low*year high*year / overlay
                             haxis=axis1
                             hminor=4
                             vaxis=axis2
                             vminor=1
                             caxis=black
                             areas=2;
run;
quit;