/* There are 3 histograms to plot a band and a center distribution.
 */
multihist() {

  TCanvas * can = new TCanvas("can", "can", 400, 300);
  can->cd();
  /*create 3 histograms */

  //this is the up limit of the band
  TH1F * h_up = new TH1F("h_up", "h_up", 50, -1, 1);
  TAxis * ay = h_up->GetYaxis();  
  ay->SetRangeUser(50, 900);
  h_up->FillRandom("gaus", 25000);
  
  //this is the center distribution
  TH1F * h_center = new TH1F("h_center", "h_center", 50, -1, 1);
  h_center->FillRandom("gaus", 20000);

  //this is the down limit of the band
  TH1F * h_down = new TH1F("h_down", "h_down", 50, -1, 1);
  h_down->FillRandom("gaus", 15000);

  h_up->SetFillColor(17);
  h_up->SetLineColor(0);
  h_down->SetFillColor(10);
  h_down->SetLineColor(0);

  //draw them on the same canvas
  //h_up->Draw("][");
  //h_down->Draw("same ][");
  //h_center->Draw("same");  
  
  THStack *hs = new THStack("hs","band");
  hs->Add(h_up);
  hs->Add(h_down);
  hs->Add(h_center);
  hs->Draw("nostack");

   
  /* below are codes to add new axis*/   
  //   TGaxis *axis = new TGaxis(ax->GetXmin(),
  // 			    ay->GetBinLowEdge(ay->GetFirst()),
  //                             ax->GetXmax(),
  // 			    ay->GetBinLowEdge(ay->GetFirst()),
  //                             ax->GetXmin(),
  //                             ax->GetXmax(),
  //                             510,"+");
  //   axis->SetName("axis");
  //   axis->SetTickSize(72);
  //   axis->SetTextFont(72);
  //   axis->Draw();   
}