Double_t erf( Double_t *x, Double_t par){
  return par*TMath::Erf(x);
}

void eff()
{
   gROOT->Reset();
   c1 = new TCanvas("c1","title",200,10,700,500);

//   c1->SetFillColor(42);
   //   c1->Range(0.0,0.0,3.5,50.0);
   c1->SetGrid();

   const Int_t n = 10;
   const Int_t ntot = 500;
   Double_t x[n], ytemp[n], y[n], ex[n], ey[n];
   Double_t erf(Double_t *, Double_t );
   
   x[0] = 0.1; ytemp[0] = 9.  ;
   x[1] = 0.2; ytemp[1] = 8.  ;
   x[2] = 0.3; ytemp[2] = 150.;
   x[3] = 0.4; ytemp[3] = 317.;
   x[4] = 0.5; ytemp[4] = 350.;
   x[5] = 0.8; ytemp[5] = 349.;
   x[6] = 1.0; ytemp[6] = 350.;
   x[7] = 2.0; ytemp[7] = 325.;
   x[8] = 3.0; ytemp[8] = 310.;
   x[9] = 5.0; ytemp[9] = 261.;
   //
   y[0] =ytemp[0]/ntot  ; ex[0] = 0.; ey[0] = sqrt(ytemp[0])/ntot;
   y[1] =ytemp[1]/ntot  ; ex[1] = 0.; ey[1] = sqrt(ytemp[1])/ntot;
   y[2] =ytemp[2]/ntot; ex[2] = 0.; ey[2] = sqrt(ytemp[2])/ntot;  
   y[3] =ytemp[3]/ntot; ex[3] = 0.; ey[3] = sqrt(ytemp[3])/ntot;  
   y[4] =ytemp[4]/ntot; ex[4] = 0.; ey[4] = sqrt(ytemp[4])/ntot;  
   y[5] =ytemp[5]/ntot; ex[5] = 0.; ey[5] = sqrt(ytemp[5])/ntot;  
   y[6] =ytemp[6]/ntot; ex[6] = 0.; ey[6] = sqrt(ytemp[6])/ntot;  
   y[7] =ytemp[7]/ntot; ex[7] = 0.; ey[7] = sqrt(ytemp[7])/ntot;  
   y[8] =ytemp[8]/ntot; ex[8] = 0.; ey[8] = sqrt(ytemp[8])/ntot;  
   y[9] =ytemp[9]/ntot; ex[9] = 0.; ey[9] = sqrt(ytemp[9])/ntot;  
   //
   gr = new TGraphErrors(n,x,y,ex,ey);
   gr->SetFillColor(19);
   gr->SetLineColor(2);
   gr->SetLineWidth(1);
   gr->SetMarkerColor(4);
   gr->SetMarkerStyle(21);
   gr->SetTitle("Track eff.");
   gr->Draw("AL*");

   // To make sure this enviornment can recognize TMath class
   //   printf("Erf(0) = %f",TMath::Erf(0.0));
   //   printf("Erf(0.5) = %f",TMath::Erf(0.5));
   //   printf("Erf(1.0) = %f",TMath::Erf(1.0));

   //
   TF1 *fitfcn = new TF1("fitfcn",erf,0.0,5.0);
   gr->Fit("fitfcn");

   //Add axis titles.
   //A graph is drawn using the services of the TH1F histogram class.
   //The histogram is created by TGraph::Paint.
   //TGraph::Paint is called by TCanvas::Update. This function is called by default
   //when typing <CR> at the keyboard. In a macro, one must force TCanvas::Update.

   c1->Update();
//   c1->GetFrame()->SetFillColor(21);
//  c1->GetFrame()->SetBorderSize(12);
   gr->GetHistogram()->SetXTitle("Pt (GeV)");
   gr->GetHistogram()->SetYTitle("eff. (# of tracks over 500 pions)");
   c1->Modified();
}