TH1 y axis drawn range

General Chat
1

Hello,
this is probably a classical request, by I couldn’t find a working solution.

I have a pretty standard TH1, that I need to draw in logy (logarithmic Y axis). I do no set any user range, maximum, minimu, just fill it and then draw it. There are no bins with 0. I draw it and everything looks good.

Now, I need to know which is the displayed Y axis range. I mean which is the minimum of the Y axis that ROOT has used to draw.

If I try: hist->GetYaxis()->GetXmin() it returns 0. Even if when displayed in the log Y scale one can see the minimum is around 0.001 (10^-3).

As it contains a p-Value I want to know from the Y axis range till wich n x sigma lines I can show (1 sigma, 2 sigma,…) in a automatic way.
canPV.pdf (13.4 KB)

Cheers
Salva

2

I think these should work

  gPad->GetFrame()->GetY1();
  gPad->GetFrame()->GetY2();

but it seems that they don’t work well for log scale.
Alternatively, you can do the opposite of what you mention, i.e. set the maximum and minimum (h->SetMaximum(somevalue); ...) by knowing how much your sigmas would add to/subtract from the highest/lowest values in the histogram (h->GetMaximum(); ...).

3

Thanks for the tip!!!

it does work, yes. :slight_smile:

In linear Y scale, gPad->GetFrame()->GetY1() returns exactly the expected value. For the log Y scale, it returns the log10 of the expected value. I mean if the Y axis starts at 0.0001, gPad->GetFrame()->GetY1() returns -4, which I can use for my calculations. Perfect :sunglasses:

Thanks again!
Beer++;
Salva

4

Great!