TH2F axis scale

ROOT
1

Hi!

How can in a TH2D the scale on both axis be set to the same value, so that if I have lets say 100 cm on one axis and 150cm on the other axis the TH2D plot is really plotted in that ratio (1 : 1.5) and not as a square? A centimeter on one axis has to have the same “length” as on the other axis…
I tried SetFixedAspectRatio and SetRangeUser but with no success…

Thanks in advance,
Stefan

2

see example in this script

[code]{
//method1. compute ymax to get a commensurate range
// 1 pixel must correspond to the same range in x and y
TCanvas c1(“c1”,“c1”,10,10,800,600);
Float_t xmin = 0;
Float_t xmax = 20;
Float_t ymin = -2;
Int_t npx = gPad->GetWw();
Int_t npy = gPad->GetWh();
Float_t ymax = ymin + (xmax-xmin)*npy/npx;
TH2F h(“h”,"",20,xmin,xmax,20,ymin,ymax);
h.Draw();
TArc a1(6,4,6);
a1.Draw();

//method2. set a square virtual canvas size in a non-square canvas
TCanvas c2(“c2”,“c2”,200,50,800,600);
c2.SetCanvasSize(700,700);
c2.DrawFrame(0,0,20,20);
TArc a2(10,10,10);
a2.Draw();
}
[/code]

Rene

3

Hi Rene!

Thanks a lot for the quick answer. In principle it seems to work, but as you might see in the attached pictures, now I have a big area where no data exists. So my question is: Can I “cut away” this from the pad? I just want to see the values up to 200 on the xaxis without loosing the scale again…

Thanks a lot,
Stefan



4

Another thing: Is it possible to have the point of origin at another place? In this case, the y values (shown above in the picture) should increase from top top bottom. So the point (0,0) should be in the upper left corner. One could say I want to “mirror” the picture along the x axis (but the x axis can be at the bottom or top…)

Cheers,
Stefan

5

Hi!

Problem seems to be solved. I just scale the gPad itself now…

But the problem with the axis remains. How can I set the origin to the left upper corner?

Cheers,
Stefan

6

Reverse axis are not provided yet. Here is an example doing it. But be careful that it invert only the axis, not the data !

reverseaxis ()
{
   TH2F *hpxpy  = new TH2F("hpxpy","py vs px",40,-4,4,40,-4,4);
   Float_t px, py;
   TRandom r;
   for (Int_t i = 0; i < 25000; i++) {
      r.Rannor(px,py);
      hpxpy->Fill(px,py);
   }
   TCanvas *c1 = new TCanvas("c1");
   hpxpy->Draw("colz");
   ReverseXAxis(hpxpy);
   ReverseYAxis(hpxpy);
}
                                                                                
ReverseXAxis (TH1 *h)
{
   // Remove the current axis
   h->GetXaxis()->SetLabelOffset(999);
   h->GetXaxis()->SetTickLength(0);
                                                                                
   // Redraw the new axis
   gPad->Update();
   TGaxis *newaxis = new TGaxis(gPad->GetUxmax(),
                                gPad->GetUymin(),
                                gPad->GetUxmin(),
                                gPad->GetUymin(),
                                h->GetXaxis()->GetXmin(),
                                h->GetXaxis()->GetXmax(),
                                510,"-");
   newaxis->SetLabelOffset(-0.03);
   newaxis->Draw();
}
                                                                                
ReverseYAxis (TH1 *h)
{
   // Remove the current axis
   h->GetYaxis()->SetLabelOffset(999);
   h->GetYaxis()->SetTickLength(0);
                                                                                
   // Redraw the new axis
   gPad->Update();
   TGaxis *newaxis = new TGaxis(gPad->GetUxmin(),
                                gPad->GetUymax(),
                                gPad->GetUxmin()-0.001,
                                gPad->GetUymin(),
                                h->GetYaxis()->GetXmin(),
                                h->GetYaxis()->GetXmax(),
                                510,"+");
   newaxis->SetLabelOffset(-0.03);
   newaxis->Draw();
}
7

What should we do if i want to reverse the data at the same time?

8

Welcome to the ROOT forum.

You can do that with TGragh.
With TH2F you will need to make a new histogram with reversed data.