Draw histogram on background

ROOT
1

Hello, I have a TCanvas from a TFile with many TGraphErrors drawn inside it. Now I want to add a TH1F inside this canvas. The question is: how to do it in a way that the histogram is behind the TGraphErrors? Is there any z-index ?

2

May be, somebody will give an idea how it’s supposed to be done in a ROOT way.
I can suggest a hack and more legal solution.

The simple but tricky way - a hack (it’s a sketch code of a terrible quality just to show an idea):

void inject()
{
   TFile * f = new TFile("data.root", "read");
   f->cd();
   
   TCanvas * c = (TCanvas *)f->Get("test_cnv");
   c->Draw();
   
   Double_t xMin = 0., xMax = 0., yMin = 0., yMax = 0.;
   c->GetRangeAxis(xMin, yMin, xMax, yMax);

   TH1F *fakeHist = new TH1F("empty", "empty", 100, xMin, xMax);
   fakeHist->FillRandom("gaus", 100000);

   TList * lst = c->GetListOfPrimitives();
   unsigned i = 0;
   for (; i < lst->GetEntries(); ++i) {
      TObject *obj = lst->At(i);
      if (obj->InheritsFrom("TGraph")) {
         lst->AddBefore(obj, fakeHist);
         break;
      }
   }

   for (i = 0; i < lst->GetEntries(); ++i) {
      TObject *obj = lst->At(i);
      if (obj->InheritsFrom("TFrame")) {
         TFrame *frame = (TFrame *)obj;
         frame->SetFillStyle(0);
         break;
      }
   }
   
   c->Update();
}

The root file is attached. Of course, this is quite an ugly and fragile solution and you’ll have to tune it to have a good picture. Another solution (which, I think, is the right and legal way) is to:

  1. Ask your canvas for its list of primitives.
  2. Create a new canvas
  3. Create your hist (probably, you’ll have to ask the first canvas about axes ranges before)
  4. newCanvas->cd(), hist->Draw()
  5. Iterate through the list of primitives and select objects you need (graphs) and draw them in the second canvas.
    data.root (14.1 KB)
3

thank you. Basically you are adding the histogram in front of the TGraph inside the list of primitives. I don’t understand the second part of your code: why do you need to change the fill style of the TFrame?

It would be nice if someone can add an option like ::Draw(“bkg”)

My present solution is much worse: I am saving the file as pdf and pushing the histogram to brackground using Inkscape

4

[quote=“wiso”]I don’t understand the second part of your code: why do you need to change the fill style of the pad?
[/quote]

I have to change a fill style for a TFrame object, otherwise, it’s white and hides a histogram.

5

I tried with my code and I have a problem with the axes: the axes of the histogram and the TGraphErrors (actually I have a TMultiGraph) are different and both are drawn on the canvas

6

I think I have solved in this way:

objs = []
for i in range(lst.GetEntries()):
    obj = lst.At(i)
    if not obj.InheritsFrom("TFrame"):
        objs.append(obj)

for obj in objs:
    lst.Remove(obj)

histo.Draw()
for o in objs:
    o.Draw()