void exampleRatioPlot() { 


   auto h1 = new TH1D("h1","h1",50,-3,3); 
   auto h2 = new TH1D("h2","h2",50,-3,3); 

   h1->FillRandom("gaus", 1000);
   h2->FillRandom("gaus", 10000);
   
   h1->Sumw2();
   h2->Sumw2();
   h1->Scale(1./h1->Integral()); 
   h2->Scale(1./h2->Integral()); 

   h1->ResetStats();
   h2->ResetStats();

   c1 = new TCanvas(); 
   c1->Divide(1,3); 
   c1->cd(1); 
   h1->Draw("HIST E"); 
   h2->SetMarkerStyle(20); 
   h2->Draw("SAME E"); 


   // compute ratio 

   c1->cd(2); 
   auto ratio = new TGraphAsymmErrors(50); 

   ratio->Divide(h1,h2,"pois midp"); 
   ratio->SetMarkerStyle(20); 

   ratio->GetXaxis()->SetRangeUser(-3,3); 
   ratio->SetMinimum(-2);
   ratio->SetMaximum(10); 
   ratio->Draw("AP");
   auto line  = new TLine(-3,1,3,1);
   line->SetLineColor(kBlue);
   line->Draw();
   ratio->SetTitle("Ratio using Poisson statistics");
   ratio->Draw("P");


   // do using TH1::Divide
   c1->cd(3); 

   auto hr = (TH1*) h1->Clone("hratio");
   hr->Reset();
   hr->Divide(h1,h2);
   hr->SetMarkerStyle(20);
   hr->SetMinimum(-2);
   hr->SetMaximum(10);
   hr->SetTitle("Ratio using Normal statistics");
   hr->Draw("E");
   
}