Exclusion band in TGraph

Dear ROOT Support,

I have a question regarding the functionality of drawing an exclusion band in TGraph as documented under
It was not clear to me from this documentation what the numbers “ff” (width of the band) was referring to. I even tried to look into the source code, but could not nest my way back to the relevant piece of code (I would appreciate it if you could point me to it). After some fiddling around, I figured that it is referring to global coordinates of the canvas (see attached code for illustration).

I was wondering whether it is possible to make it refer to the coordinates of the y-axis instead? Otherwise, I don’t really see how it is useful, but perhaps I am missing something.

tgraphexclusion.C (622 Bytes)

It is some kind of absolute value not referring to anything. It is like the line width just a number … the bigger it is the wider the exclusion shade is … do not try to use it as a precise filled polygon.

Thanks for the response.
Okay, so I don’t see how this ever would be useful.

You are miss using it. It is meant to show an exclusion zone, not to define an hatched polygon. If you want an hatched polygon do an hatched polygon not an exclusion graph.

What I want is to show an exclusion area. Clearly my definition of exclusion area does not match the definition used here. I would be glad to learn in which occasions one should use the functionality of exclusion band on a TGraph.

The exclusion area in ROOT are meant to hatch one side of a graph to show that the data on that side are excluded. May be explain what you really want ?

That is exactly what I want.
But unless it is possible to link the width of the area to the y-axis, I can’t see how it makes sense. If the area is not referring to any coordinates, how can it show a (meaningful) excluded region? I have a limit (y as function of x), and I want to make the exclusion area from the limit up to three times the limit (3*y).

So that’s not what the exclusion graph is doing. As I said, it just draw hatches on one side and the width of it is just a number. What you want seems to be some precise hatches area… in that case it is a polygon.

I will use a TGraphAsymmErrors. It is still not clear to me when the ‘exclusion band’ option of TGraph would be useful.

Many people use it and are happy with it. The specs were done according to users’ request.
I think you simply use the wrong object for what you want to do.

Yes, that is clear now. I’ve been trying to ask for an explanation of when people find it useful, so I can learn.