void plot_qbad(){
  double p[7]={0.5,1.0,1.5,2.0,2.5,3.0,3.5};
  //  double qbad[7]={0.02,0.047,0.072,0.099,0.106,0.118,0.138};
  double qbad[7]={0.019,0.040,0.063,0.088,0.093,0.106,0.127};
 
  TH2F *hr13 = new TH2F("hr13","Charge determination",
                        2,0,4.,2,0,0.2);
  hr13->SetXTitle("p (GeV/c)");
  hr13->SetYTitle("fraction of segments with wrong charge");
  hr13->Draw();

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

  g0->Draw("p");
}