void quantiles() {
   // demo for quantiles
   // Author; Rene Brun
   const Int_t nq = 100;
   const Int_t nshots = 10;
   Double_t xq[nq];  // position where to compute the quantiles in [0,1]
   Double_t yq[nq];  // array to contain the quantiles
   for (Int_t i=0;i<nq;i++) xq[i] = Float_t(i+1)/nq;

   TGraph *gr70 = new TGraph(nshots);
   TGraph *gr90 = new TGraph(nshots);
   TGraph *gr98 = new TGraph(nshots);
   TH1F *h = new TH1F("h","demo quantiles",50,-3,3);
   
   for (Int_t shot=0;shot<nshots;shot++) {
      h->FillRandom("gaus",50);
      h->GetQuantiles(nq,yq,xq);
      gr70->SetPoint(shot,shot+1,yq[70]);
      gr90->SetPoint(shot,shot+1,yq[90]);
      gr98->SetPoint(shot,shot+1,yq[98]);
   }
   
   //show the original histogram in the top pad
   TCanvas *c1 = new TCanvas("c1","demo quantiles",10,10,600,900);
   c1->SetFillColor(41);
   c1->Divide(1,3);
   c1->cd(1);
   h->SetFillColor(38);
   h->Draw();
   
   // show the final quantiles in the middle pad
   c1->cd(2);
   gPad->SetFrameFillColor(33);
   gPad->SetGrid();
   TGraph *gr1 = new TGraph(nq,xq,yq);
   gr1->SetTitle("final quantiles");
   gr1->SetMarkerStyle(21);
   gr1->SetMarkerColor(kRed);
   gr1->SetMarkerSize(0.3);
   gr1->Draw("ap");

// FIT interval [0, 1]!!!
   Double_t par0, par1;
   TF1 *f1 = new TF1("f1", "pol1", 0, 1);
   gr1->Fit("f1", "RQ");
   par0 = f1->GetParameter(0);
   par1 = f1->GetParameter(1);
cout << "------ interval [0, 1] ------" << endl;
cout << "par0_f1 = " << par0 << endl;
cout << "par1_f1 = " << par1 << endl;
   
   // show the final quantiles in the middle pad
   c1->cd(3);
   gPad->SetFrameFillColor(33);
   gPad->SetGrid();
   TGraph *gr2 = new TGraph(nq,xq,yq);
   gr2->SetTitle("final quantiles");
   gr2->SetMarkerStyle(21);
   gr2->SetMarkerColor(kRed);
   gr2->SetMarkerSize(0.3);
   gr2->Draw("ap");

// FIT interval [0.3, 0.7]!!!
   TF1 *f2 = new TF1("f2", "pol1", 0.3, 0.7);
   gr2->Fit("f2", "RQ");
   par0 = f2->GetParameter(0);
   par1 = f2->GetParameter(1);
cout << "------ interval [0.3, 0.7] ------" << endl;
cout << "par0_f2 = " << par0 << endl;
cout << "par1_f2 = " << par1 << endl;
}