Handles in Graph Builder

[#1]

I'm trying to understand Handles, especially in Graph Builder.

First of all, they don't seem to work.  But beyond that, I don't understand how they could work -- if I enter a Handle() as a script within Graph Builder's "Customize..." Customize Graph dialog, does it execute that script every time the graph is redrawn?  If so, is it creating many instances of a Handle object?

I thought this script attached to Big Class would give me a circle that I could drag around the plot, but it doesn't work:

Graph Builder(
	Size( 897, 611 ),
	Variables( X( :age ), Y( :height ) ),
	Elements( Points( X, Y, Legend( 8 ) ) ),
	SendToReport(
		Dispatch(
			{},
			"height",
			ScaleBox,
			{Min( 29.6266879535559 ), Max( 70.912 ), Inc( 10 ), Minor Ticks( 1 )}
		),
		Dispatch(
			{},
			"Graph Builder",
			FrameBox,
			{Reference Line Order( 3 ),
			Add Graphics Script(
				2,
				Description( "" ),
				Handle( 12, 56, Circle( {x, y}, 3 ) )
			)}
		)
	)
)

What am I doing wrong?

[#2]

This gives an example how Handle works

Names Default To Here(1); 

dt = Open("$SAMPLE_DATA/Big Class.jmp");

exx = 3;
exy = 56;

gb = dt << Graph Builder(
	Size(897, 611),
	Variables(X(:age), Y(:height)),
	Elements(Points(X, Y, Legend(8))),
	SendToReport(
		Dispatch(
			{},
			"height",
			ScaleBox,
			{Min(29.6266879535559), Max(70.912), Inc(10), Minor Ticks(1)}
		),
		Dispatch(
			{},
			"Graph Builder",
			FrameBox,
			{Reference Line Order(3), Add Graphics Script(
				2,
				Description(""),
				Handle(
					exx,
					exy,
					exx = x;
					exy = y;
				);
				show(exx, exy);
				Circle({exx, exy}, 3);
			)}
		)
	)
);