how to draw a line in the plot

shiyiming

I use

proc gplot;
plot a*b/overlay;
run;



Now I have  a question. I want to put a horizontal line in vaxis, is there any statement which can make it?

shiyiming

data xy;
input a b;
cards;
1 2
2 3
3 4
;
run;
symbol1 i=j;
axis1 order=(0 to 5);
proc gplot data=xy;
    plot a*b/vref=2 vaxis=axis1;
run;