You could use a graphics script. In the sample case, find 'HLine()' in 'Help > Scripting Index'. For eaxmple:
is created by this JSL:
NamesDefaultToHere(1);
dt = Open("$SAMPLE_DATA/Big Class.jmp");
gb = dt << Graph Builder(
Size( 529, 453 ),
Show Control Panel( 0 ),
Show Legend( 0 ),
Variables( X( :weight ), Y( :height ) ),
Elements( Points( X, Y, Legend( 6 ) ), Smoother( X, Y, Legend( 7 ) ) ),
SendToReport(
Dispatch(
{},
"Graph Builder",
FrameBox,
{Add Graphics Script(
1,
Description( "LowerSpec" ),
Pen Color( "Red" );
H Line( 100, 140, 60 );
), Add Graphics Script(
3,
Description( "UpperSpec" ),
Pen Color( "Red" );
H Line( 100, 140, 67 );
), Grid Line Order( 2 ), Reference Line Order( 4 )}
)
)
);
Once you have the plot, right click on the graphics box and select 'Customize...'.