Example of drawing overlaid histograms from two TTrees in two TFiles

overlay.C

// Draw overlaid histograms from a RAT ntuple (for example).
//
// Usage:
//   $ root -l -x 'overlay.C("filename1.root", "filename2.root")'
//
// The trick is that histograms are associated with a TDirectory
// upon creation, which is most likely the open file. When the
// directory changes when a new file is opened, the pointer to
// the histogram is invalidated. TH1::SetDirectory is used to
// detach a histogram from a file.

void overlay(TString file1, TString file2) {
  TFile f1(file1);
  TTree* t1 = (TTree*) f1.Get("output");
  TH1F* h1 = new TH1F("h1", "nhits", 100, 0, 2000);
  t1->Draw("nhits>>h1", "", "goff");
  h1->SetDirectory(0);  // Trick: detach histogram from the file!
  f1.Close();

  TFile f2(file2);
  TTree* t2 = (TTree*) f2.Get("output");
  TH1F* h2 = new TH1F("h2", "nhits", 100, 0, 2000);
  t2->Draw("nhits>>h2", "", "goff");
  h2->SetDirectory(0);
  f2.Close();

  TCanvas* c = new TCanvas();
  h1->Scale(1.0 / h1->Integral());
  h1->Draw();

  h2->Scale(1.0 / h2->Integral());
  h2->SetLineColor(kRed);
  h2->Draw("same");
}