I can assume that the issue falls outside of the code you provided. Here is a simple example, which when run against the Sample Data table called Big Class works fine in JMP 15
Graph Builder(
Size( 528, 456 ),
Show Control Panel( 0 ),
Legend Position( "Inside Left" ),
Variables( X( :weight ), Y( :height ), Overlay( :sex ) ),
Elements( Points( X, Y, Legend( 12 ) ), Smoother( X, Y, Legend( 13 ) ) ),
SendToReport( Dispatch( {}, "400", LegendBox, {Set Title( "" )} ) )
);
See if it works for you, and then check your complete code for possible issues
Jim