Here is a simple example that draws a reference line based upon the value of row 1 from column Weight

Graph Builder(
Variables( X( :weight ), Y( :height ) ),
Elements( Points( X, Y, Legend( 6 ) ), Smoother( X, Y, Legend( 7 ) ) ),
SendToReport(
Dispatch( {}, {:weight}, ScaleBox,
{Add Ref Line( 95, "Solid", "Red", "", 1 )}
),
Dispatch( {}, "Graph Builder", FrameBox,
{Add Graphics Script(
2,
Description( "" ),
rp = Current Report();
rp[axis box( 1 )] << add ref line( :weight[1], "solid", red );
)}
)
)
);
I produced the complete JSL by right clicking on the graph and selecting Customize. Then clicking on the "+" and writing the simple script
rp=current report();
rp[axis box(1)] << add ref line(:weight[1],"solid",red)

After applying the little script, I went back to the red triangle in the graph, and saved the script to a script window.
Jim