How to draw a trapezoid inside Fit Y by X chart or graph builder?

[#1]

I am looking a way to draw a trapezoid limit inside Fit Y by X chart or graph builder like excel example below. Any idea? 

[#2]

Sorry, I do not know off hand, so I'll let others chime in. What is the criteria you want to use to create the trapezoid? Why do you want this?

[#3]

Hi,
Could you confirm that you are looking for a method to calculate the anchor points of your trapezoid and not just a method to draw the trapezoid which can be easily done with the Polygon () function? If it is the case, what are the criteria for the desired dimensions of your trapezoid (i.e. anchor point positions)?
Best,
TS

[#4]

If you right click on the graph, you can Customize the graph, which will allow you to add the graphics you want to the chart.

Here is what the customization window looks like

And here is the simple graphic code required

pen color("blue");
pen size(2);
line([.273, .292, .339,.319,.273 ], [.279, .334, .359,.302,.279]);
Marker Size( 5 );
		Marker(
			color state( blue ),
			Marker State( 15 ),
			[.273, .292, .339,.319],
			[.279, .334, .359,.302]
		);

[#5]

@OneNorthJMP, Perhaps you will find what you are looking for in the Scripting Index, under the Help menu, by searching Polygon. You should be able to combine what you see under polygon in the scripting index with graph builder by right clicking in the graph space and selecting customize, then clicking the + symbol in the dialog that appears, to add a custom script (polygon()).
Hope this helps.