void
ptdrHistogramStyle()
{

  if(gSystem->AccessPathName("ptdrHistogramStyle_C.so")!=0){
    cout << "The shared library ptdrHistogramStyle_C.so not found." << endl
	 << "To get it from ptdrHistogramStyle.C, type in ROOT \".L ptdrHistogramStyle.C++\"" << endl;
    gSystem->Exit(0);
  }
  gSystem->Load("ptdrHistogramStyle_C.so");

  TString varname="ptj1";
  Int_t nBins=50;
  Double_t min=0;
  Double_t max=200;
  TPTDRHistogramStyle* hs = new TPTDRHistogramStyle;

  /** From ptdrHistogramStyle.h :
      TPTDRHistogramStyle::Add(TString variableName, TString fileName, TString lName,
			    Int_t nBins=100, Double_t min=0, Double_t max=0, 
			    Double_t weight=1, TString treeName="T");

      reads variableName from the treeName from the fileName and
      puts it to the array of histograms. lName is legend name for the
      histogram. nBins, min and max define the number of bins, 
      maximal and minimal values for the X-axis(range). weight can be
      used when stacking histograms with different weights. */
  hs->AddHistogram(varname,"schannel.root","sPtj1",nBins,min,max);
  hs->AddHistogram(varname,"tchannel.root","tPt1",nBins,min,max);
  hs->AddHistogram(varname,"ttbar.root","ttbarPtj1",nBins,min,max);
  hs->AddHistogram(varname,"wbb.root","wbbPtj1",nBins,min,max);
  
  varname="ptj2";
  hs->AddHistogram(varname,"schannel.root","sPtj2",nBins,min,max);
  hs->AddHistogram(varname,"tchannel.root","tPtj2",nBins,min,max);

  cout << "Plotting " << hs->GetHists()->GetSize() << " histograms..." << endl;
  new TCanvas("Pt","Pt",0,0,600,400);

  hs->DrawHistograms();
  hs->DrawLegend();
}