void plot_ptres(){
  double p[7]={0.5,1.0,1.5,2.0,2.5,3.0,3.5};
  double dpt_corr[7]={0.064,0.066,0.076,0.090,0.113,0.133,0.138};
  double dpt_uncorr[7]={0.08,0.106,0.179,0.221,0.258,0.345,0.381};

  TH2F *hr13 = new TH2F("hr13","Transverse momentum resolution from package 1",
                        2,0,4.,2,0,0.4);
  hr13->SetXTitle("p (GeV/c)");
  hr13->SetYTitle("#sigma(#deltapT/pT)");
  hr13->Draw();

  TGraph *g0=new TGraph(7,p,dpt_corr); 
  g0->SetMarkerStyle(kFullCircle);
  g0->SetMarkerColor(1);
  g0->SetLineColor(1);

  g0->Draw("p");  

  TGraph *g1=new TGraph(7,p,dpt_uncorr); 
  g1->SetMarkerStyle(kOpenCircle);
  g1->SetMarkerColor(1);
  g1->SetLineColor(1);

  g1->Draw("p");

  TLegend *legend=new TLegend(0.25,0.75,0.6,0.85);
  legend->AddEntry(g0,"Lorentz-effect correction","p");
  legend->AddEntry(g1,"No Lorentz-effect correction","p");
  legend->Draw();

}