I could not see any option to directly change the borders around each frame box, so here is the method that I came up with:
Names Default To Here( 1 );
dt = Open( "$SAMPLE_DATA/big class.jmp" );
gb = Graph Builder(
Size( 570, 578 ),
Variables( X( :sex ), Y( :height ), Group Y( :age ) ),
Elements( Smoother( X, Y, Legend( 1 ) ) )
);
(Report( gb ) << xpath( "//FrameBox" )) << add graphics script(
Pen Size( 2 );
Pen Color( red );
H Line( -5000, 5000, Report( gb )[axisbox( 2 )] << get min );
);
Jim