One option is to create one additional column to your table which indicates the maximum value possible and then create a line chart using that with fill below enabled.
 
Column 7 has just value 70 in all rows

Graph Builder(
	Size(570, 798),
	Variables(
		X(:weight),
		Y(:height),
		Y(:Column 7, Position(1)),
		Group X(:age),
		Overlay(:than),
		Color(:sex)
	),
	Elements(
		Bar(X, Y(1), Legend(5), Response Axis("X")),
		Line(Y(2), Color(0), Legend(8), Fill("Fill Below"))
	),
	SendToReport(
		Dispatch({"Bar"}, "", OutlineBox, {Close(0)}),
		Dispatch({"Line"}, "", OutlineBox, {Close(0)}),
		Dispatch(
			{},
			"height",
			ScaleBox,
			{Min(0), Max(70.3382899628253), Inc(10), Minor Ticks(0)}
		),
		Dispatch(
			{},
			"400",
			ScaleBox,
			{Legend Model(
				8,
				Properties(0, {Line Color(35)}, Item ID("0", 1)),
				Properties(-1, {Line Color(35)}, Item ID("Mean(Column 7)", 1))
			)}
		)
	)
)
					
				
			
			
				
	-Jarmo