The first plot request plots High on the vertical axis, plots Year on the horizontal axis, and specifies an asterisk as a plotting symbol. The second plot request plots Low on the vertical axis, plots Year on the horizontal axis, and specifies an ' o ' as a plotting symbol. OVERLAY superimposes the second plot onto the first. BOX draws a box around the plot. OVERLAY and BOX apply to both plot requests. 
proc plot data=djia;
   plot high*year='*'
        low*year='o' / overlay box;
   title 'Plot of Highs and Lows';
   title2 'for the Dow Jones Industrial Average';
run;