I think you have to do this through legend server (or when building the graph builder expression)
Names Default To Here(1); 
dt = open("$SAMPLE_DATA/Big Class.jmp");
gb = dt << Graph Builder(
	Transform Column("Row", Formula(Row())),
	Variables(X(:Row), Y(:height)),
	Elements(Line Of Fit(X, Y, Legend(5), Confidence of Fit(0))),
	SendToReport(
		Dispatch({}, "400", ScaleBox,
			{Legend Model(
				5,
				Properties(
					0,
					{Line Label Properties({Name Label(1)})},
					Item ID("height", 1)
				)
			)}
		)
	)
);
gb << Add Variable({:weight, Role("Y"), Position(1)});
server = gb << Get Legend Server;
items = server << Get Legend Items;
For(i = 1, i <= N Items(items[1]), i++,
	items[1][i] << Set Properties(
		{Line Label Properties({Name Label(1)})}
	)
);
					
				
			
			
				
	-Jarmo