Automatically find maximum of several histogramms in TCanvas

ROOT
1

Dear Rooters,

a problem I stumble across ever so often:

Drawing multiple histogramms into a Canvas, the first histogram will define the axis range in both x & y.

Is there an elegant way – I know how to do it non-elegantly – to set the maximum(minimum) of the y-axis to the highest maximum from all histograms, such that all histograms are completely visible at the same time without further need for readjusting scales?

Best regards,

Erik

2

Use a THStack. See: $ROOTSYS/tutorials/hist/hstack.C

1 Like
3

Somehow I am getting in trouble since I want to read histograms from files from a TChain and adding them to the stack as shown below, the code always produces a seg fault

TChain *chain = new TChain();
chain->Add("somefilename*.root");
TObjArray *fileElements=chain->GetListOfFiles();
TIter next(fileElements);
TChainElement *chEl=0;
THStack *stack = new THStack("name","title");
 while (( chEl=(TChainElement*)next() )) {
    TFile f(chEl->GetTitle());
    TKey* k_gain = f.FindKeyAny("histoname");
    TH1* h_gain = static_cast<TH1*>(k_gain->ReadObj());
    stack->Add(h_gain);

}

stack->Draw("nostack");

It seems to me that the objects disappear when the file is closed, even when introduce another Clone statement.
The problem did not arise when I just used the histograms since I made a DrawCopy() . Probably something stupid I am overlooking.

Any hint is appreciated.

Erik

4

Add the line marked with <====== to your code

TChain *chain = new TChain();
chain->Add("somefilename*.root");
TObjArray *fileElements=chain->GetListOfFiles();
TIter next(fileElements);
TChainElement *chEl=0;
THStack *stack = new THStack("name","title");
 while (( chEl=(TChainElement*)next() )) {
    TFile f(chEl->GetTitle());
    TKey* k_gain = f.FindKeyAny("histoname");
    TH1* h_gain = static_cast<TH1*>(k_gain->ReadObj());
     h_gain->SetDirectory(0); //<================
    stack->Add(h_gain);

}

stack->Draw("nostack");

Rene

5

Hi Rene,

the line you indicate indeed solved the problem, thanks.

Somehow I keep forgetting about the directory association of the histograms.

Best regards

Erik

6

How do I resolve the same problem if I draw a THStack, a TGraph and a TH1 on the same canvas?

7

https://root.cern/doc/master/classTHStack.html#a76deae09231652a9e1c3e65fb5cdc3c5

8

@couet, it would be nice if the canvas was intelligent and would automatically adjust its own maximum depending on what gets drawn on it.

9

It is exactly what it does when you Draw a THStack.