How to re-apply a zoom to an histogram

Hi,

I have an histogram (h1) and the user can zoom on it (click drag on the x or y axis). From time to time I get a new histogram (h2 per instance) to replace h1. I would like to have the same viewing range in h2 than in h1.

First question : how to get the viewing range ?
I see GetMaximum in the TH1 class but this is only for axis Y.
There is GetXMin in the TAxis class, but it doesn’t seem to return the viewing range.
Finally, I guess I should use GetUxmin, GetUxmax, etc… methods from the TPad, but I am not entirely sure.

Second question : How to change the viewing range of h2 ?
Using the Range method of TPad ? the TH1::SetMinimum ? or the TAxis::SetRange ?

Thanks in advance for any explanation on the differences between those methods.

Cheers,

Barth

On the Y axis you can do:

root [0] hpx->GetMinimum()
(const Double_t)0.00000000000000000e+00
root [1] hpx->SetMinimum(-10)
root [2] hpx->GetMinimum()   
(const Double_t)(-1.00000000000000000e+01)
root [3] 

Same thing for the Maximum.

On the X axis

root [4] TAxis *x = hpx->GetXaxis()
root [5] x->GetXmin()   
(const Double_t)(-4.00000000000000000e+00)
root [6] x->SetLimits(-20,10)
root [7] x->GetXmin()        
(const Double_t)(-2.00000000000000000e+01)
root [8] 

Hi !

Sorry for the late reply.

Your solution for the Y axis works perfectly.
However the solution for the X axis works to set new limits but what about retrieving the limits values of an histogram ?

Here is an example of what I want :

root [0] new TCanvas
(class TCanvas*)0x873dc50
root [1] h = new TH1F("test", "test", 20, 0, 20)
(class TH1F*)0x87b8ad8
root [2] h->Draw()
root [3] h->Fill(4)
(Int_t)(5)
root [4] // here you manually zoom on a part of X axis
root [5] TAxis *x = h->GetXaxis()
root [6] x->GetXmin()
(const Double_t)0.00000000000000000e+00

After the zoom, I want to get the new limits but I still get 0.0.

Any clue ?

Thanks for your help

Barth

Sorry for the late answer, I was away.

What about:

TAxis *ax = hpx->GetHistogram()->GetXaxis();
Double_t x1 = ax->GetBinLowEdge(ax->GetFirst());
Double_t x2 = ax->GetBinUpEdge(ax->GetLast());

?

Hi !

No problem. I should have replied myself with mypartial solution.
I finally do the following to get range of axes X and Y with and without zoom:

// this gives you the value taking into account the zoom
Double_t xmin = pad->GetUxmin(); 
Double_t xmax = pad->GetUxmax();
Double_t ymin = pad->GetUymin(); 
Double_t ymax = pad->GetUymax();

// and this gives you the "real" value of the axis (no zoom)
Double_t xminLimit = histo->GetXaxis()->GetXmin();  
Double_t xmaxLimit = histo->GetXaxis()->GetXmax();
Double_t yminLimit = histo->GetMinimum();
Double_t ymaxLimit = histo->GetMaximum();

I don’t know if it is the correct way of doing it, but it works. And I understand why the zoom values are stored in the pad, as you can have the same histogram displayed in two different pads.

However, I don’t understand why there is no common way of retrieving axes extremities (histo->GetXaxis()->GetXmin(); and histo->GetMinimum(); ). This is missleading.
Moreover, the synthax histo->GetXaxis()->GetXmin() leads you to search for a histo->GetYaxis()->GetYmin() which doesn’t exist. You end up with histo->GetYaxis()->GetXmin() which is non-sense and doesn’t work.

Now, what bothers me is how to set the zoom. I don’t want to set it on the histogram but on the pad (again, if I display the histogram in 2 pads, I want to put different values on each of them). I was looking for something like SetUymin(), as I used GetUymin(), but it simply doesn’t exist. How should I do it ?

thanks for your help !

Barth

example:

   hpx->GetXaxis()->SetRange(33,71);

Hi !

I am sorry but that doesn’t solve the problem I was mentionning: 1 histogram displayed on 2 pads (A and B). If you use histo->SetRange or whatever method on the histogram, it will be applied on both pads.
The zoom should belong to the TPad and not to the histogram. And I am sure it is the case as you can have different zoom in A and B when setting the zoom manually.

Do you know if there is a method equivalent to GetUxmin (and others) but to set these values on the pad ?

Thanks,

Barth

Ah yes. Sorry.

The zoom belong to the histogram. This will not be change. To draw the same histogram on two different pads and make them independent use DrawClone()

Hi,

Ok, I will try that.

I know it won’t be changed ; but this is clearly inconsistent with other methods such as setDrawOptions or setLogx which belong to TPad… :slight_smile:

Thanks for your help and have a nice day !

Barth