DrawClonePad() removes all axis formatting

ROOT
1

Hi all!

I have an issue when combining a bunch of plots onto one TCanvas. Basically, I have a few macros that each generate a plot, with heavily altered axis labels (e.g. removing first/last tick, changing numbers to pi etc).

I have another macro that takes all these plots, and draws them into one TCanvas on different TPads, in a specific “formation”. I plot them on different TPads with DwarClonePad(), but it seems to remove the nice axis formatting…

void plotter(){

    // Example plot - in reality I have 10 different macros that
    // make plots which I then need to combine into one
    TCanvas *c = new TCanvas("c", "c", 400, 400);
    Double_t pi = TMath::Pi();
    TF1*   f = new TF1("f","TMath::Cos(x/TMath::Pi())", -pi, pi);
    TH1*   h = f->GetHistogram();
    TAxis* a = h->GetXaxis();
    a->SetNdivisions(-502);
    // Some axis customization
    a->ChangeLabel(1,-1,-1,-1,-1,-1,"-#pi");
    a->ChangeLabel(-1,-1,-1,-1,-1,-1,"#pi");
    f->Draw();
    // This is what I want the example axis to look like
    c->SaveAs("correct_axis.pdf");

    // Big multi-plot canvas - this is where I will combine all the
    // plots together. In this example I'm plotting one TCanvas
    // twice, in my big macro I'm loading 10 files with a TCanvas each
    TCanvas *big = new TCanvas("big", "big", 1000, 500);

    // PAD 1
    big->cd();
    TPad *left = new TPad("left", "left", 0.1, 0.1, 0.5, 0.9);
    left->Draw();
    left->cd();
    c->DrawClonePad();

    // PAD 2
    big->cd();
    TPad *right= new TPad("right", "right", 0.5, 0.1, 0.9, 0.9);
    right->Draw();
    right->cd();
    c->DrawClonePad();
    
    // The X axis in this plot is changed, and I need them to be
    // the same as in correct_axis.pdf
    big->SaveAs("bad_axis.pdf");
  }

Does anyone know how to go around this?

ROOT Version: 6.18/04
Platform: Gentoo, kernel: 5.1.6
Compiler: gcc 9.1.0
Python: 3.6.5

2

Yes I see the Changed-Labels are lost. Very like TAxis has been recreated.
I need to investigate.

3

I simplified the reproducer:

void plotter(){
   TCanvas *c = new TCanvas("c", "c", 400, 400);
   auto h = new TH1F("h","h",100,-1,1);
   TAxis* a = h->GetXaxis();
   a->SetNdivisions(-502);
   a->ChangeLabel(1,-1,-1,-1,-1,-1,"-#pi");
   a->ChangeLabel(-1,-1,-1,-1,-1,-1,"#pi");
   h->Draw();

   TCanvas *c2 = new TCanvas("c2", "c2", 400,0,400, 400);
   c2->cd();
   c->DrawClonePad();
}
4

Good to know it’s not me doing something stupid.
For now I went around this problem by iterating through all the canvas primitives and drawing them directly onto a TPad (heavily modified to look like the canvas I wanted to draw with), but in case of my code it’s just an ugly hack that might won’t work for all of my plots in the future.

5

I need to understand why the modified labels are lost during the cloning pad operation.

6

We can reduce it to:

void plotter(){
   TCanvas *c = new TCanvas("c", "c", 400, 400);
   auto h = new TH1F("h","h",100,-1,1);
   TAxis* a = h->GetXaxis();
   a->SetNdivisions(-502);
   a->ChangeLabel(1,-1,-1,-1,-1,-1,"-#pi");
   a->ChangeLabel(-1,-1,-1,-1,-1,-1,"#pi");
   h->Draw();

   TCanvas *c2 = new TCanvas("c2", "c2", 400,0,400, 400);
   h->Clone()->Draw();
}

So the Clone() method is the culprit.

7

The modified labels were not copied in TAxis::Copy. Copying them makes it work for histograms.
But TF1:Clone is inherited from TObject::Clone which does keep the internal histograms. So the fix in TAxis::Copy does not fix the issue for TF1.

8

I made a JIra ticket for this issue. I will submit a PR soon.

9

With this PR your macro gives me the following plot:

59
Screenshot 2020-02-20 at 10.36.59.jpg1548×832 115 KB

10

The PR fixing this issue has been pushed to master.

11

Thank you for solving this bug!