ROOT37: histogram画图

void ana3(){

  // read the TFile WFM_Analysis.root

  TFile *file = new TFile( "WFM_Analysis.root", "read" );

  TTree *tree = (TTree*)file->Get("WFM");

  // get data 

  const int N = tree->GetEntries();

  double timestamp, energy1, energy2, time1, time2;

  bool flag_coin;

  tree->SetBranchAddress( "TimeStamp", &timestamp );

  tree->SetBranchAddress( "PeakEnergy1", &energy1 );

  tree->SetBranchAddress( "PeakEnergy2", &energy2 );

  tree->SetBranchAddress( "PeakTime1", &time1 );

  tree->SetBranchAddress( "PeakTime2", &time2 );

  tree->SetBranchAddress( "Flag_Coin", &flag_coin );

  

  // plot

  TH1D *hgamma1 = new TH1D( "hgamma1", "Energy distribution of gamma1", 200, 300, 700 );

  TH1D *hlifetime = new TH1D( "hlifetime", "Lifetime of ep annihilation", 100, 0, 20 );

  

  for( int i=0; i<N; i++ ){

    tree->GetEntry( i );

    hgamma1->Fill( energy1 );

    if( flag_coin == 1 ){

      hlifetime->Fill( timestamp );

    }

  }

  TF1 *f1 = new TF1( "f1", "[0]*exp(-x/[1])+[2]", 0, 20 );

  f1->SetParameters( 50, 3, 1 );

  f1->SetLineWidth( 2 );

  

  TCanvas *c1 = new TCanvas();

  hgamma1->Draw();

  TCanvas *c2 = new TCanvas("c2", "lifeitme", 1200, 500);

  c2->Divide( 2, 1 );

  c2->cd(1);

  hlifetime->SetStats(0);

  hlifetime->SetMarkerStyle( 8 );

  hlifetime->SetMarkerColor( 1 );

  hlifetime->SetLineWidth( 2 );

  hlifetime->GetXaxis()->SetTitle( "Time [#mus]" );

  hlifetime->GetYaxis()->SetTitle( "ep annihilation / 0.2#mus" );

  hlifetime->GetYaxis()->SetRangeUser( 0, 200 );

  

  hlifetime->Draw("ep");

  hlifetime->Fit( "f1" );

  TLegend *leg = new TLegend( 0.6, 0.7, 0.94, 0.89 );

  leg->AddEntry( hlifetime, "Experiment Data" );

  leg->AddEntry( f1, "Exponential Fitting" );

  leg->Draw("same");

  double chisq = f1->GetChisquare();

  int ndf = f1->GetNDF();

  TPaveText *text = new TPaveText( 0.4, 0.4, 0.7, 0.6, "NDC");

  text->SetTextAlign( 12 );

  text->AddText( "ep#rightarrow #gamma#gamma" );

  text->AddText( Form("#chi^{2}/ndf = %.02f/%d", chisq, ndf) );

  text->AddText( Form("Lifetime = %.02f #pm %.02f [#mus]", f1->GetParameter(1), f1->GetParError(1) ));

  text->Draw( "same" );

  c2->cd(2);

  hlifetime->SetFillColor( 8 );

  hlifetime->Draw();

  leg->Draw("same");

  text->Draw( "same" );

  c2->Print( "lifetime.pdf" );

}