transpad.C: Example of a canvas showing two histograms with different scales.

// Example of a canvas showing two histograms with different scales.
// The second histogram is drawn in a transparent pad
//Author: Rene Brun
   
void transpad() {
   TCanvas *c1 = new TCanvas("c1","transparent pad",200,10,700,500);
   TPad *pad1 = new TPad("pad1","",0,0,1,1);
   TPad *pad2 = new TPad("pad2","",0,0,1,1);
   pad2->SetFillStyle(4000); //will be transparent
   pad1->Draw();
   pad1->cd();

   TH1F *h1 = new TH1F("h1","h1",100,-3,3);
   TH1F *h2 = new TH1F("h2","h2",100,-3,3);
   TRandom r;
   for (Int_t i=0;i<100000;i++) {
      Double_t x1 = r.Gaus(-1,0.5);
      Double_t x2 = r.Gaus(1,1.5);
      if (i <1000) h1->Fill(x1);
      h2->Fill(x2);
   }
   h1->Draw();
   pad1->Update(); //this will force the generation of the "stats" box
   TPaveStats *ps1 = (TPaveStats*)h1->GetListOfFunctions()->FindObject("stats");
   ps1->SetX1NDC(0.4); ps1->SetX2NDC(0.6);
   pad1->Modified();
   c1->cd();
   
   //compute the pad range with suitable margins
   Double_t ymin = 0;
   Double_t ymax = 2000;
   Double_t dy = (ymax-ymin)/0.8; //10 per cent margins top and bottom
   Double_t xmin = -3;
   Double_t xmax = 3;
   Double_t dx = (xmax-xmin)/0.8; //10 per cent margins left and right
   pad2->Range(xmin-0.1*dx,ymin-0.1*dy,xmax+0.1*dx,ymax+0.1*dy);
   pad2->Draw();
   pad2->cd();
   h2->SetLineColor(kRed);
   h2->Draw("][sames");
   pad2->Update();
   TPaveStats *ps2 = (TPaveStats*)h2->GetListOfFunctions()->FindObject("stats");
   ps2->SetX1NDC(0.65); ps2->SetX2NDC(0.85);
   ps2->SetTextColor(kRed);
   
   // draw axis on the right side of the pad
   TGaxis *axis = new TGaxis(xmax,ymin,xmax,ymax,ymin,ymax,50510,"+L");
   axis->SetLabelColor(kRed);
   axis->Draw();
}