void GunMagnet_Bz(){
  
  gROOT->Reset();
  
#define MAX_nentries 10000;
  
  TString pstit;
  
  // define style here 
  // general parameters
  gStyle->SetOptDate(0);     gStyle->SetOptTitle(0);
  gStyle->SetStatColor(10);  gStyle->SetStatH(0.1);
  gStyle->SetStatW(0.2);     gStyle->SetOptStat(1); 
  
  gStyle->SetOptFit(0); 
  
  // canvas parameters
  gStyle->SetFrameBorderMode(0);
  gStyle->SetFrameBorderSize(0);
  gStyle->SetFrameFillColor(0);
  gStyle->SetCanvasColor(0);
  
  // pads parameters
  //  gStyle->SetPadColor(39); 
  gStyle->SetPadColor(0); 
  gStyle->SetPadBorderMode(0);
  gStyle->SetPadBorderSize(0);
  gStyle->SetPadBottomMargin(0.18);
  gStyle->SetPadRightMargin(0.10);
  gStyle->SetPadLeftMargin(0.14);
  gStyle->SetLabelSize(0.04,"x");
  gStyle->SetLabelSize(0.04,"y");
  gStyle->SetTitleXSize(0.08);
  gStyle->SetPaperSize(10,12);
  
  gStyle->SetOptLogx(0);
  gStyle->SetOptLogy(0);
  
  gStyle->SetTitleYOffset(0.8);
  gStyle->SetTitleYSize(0.10);
  gROOT->ForceStyle();  
  
  //
  
  Double_t zc[MAX_nentries], zp[MAX_nentries];
  Double_t x[MAX_nentries], y[MAX_nentries];

  Double_t Bz_coil_puck[MAX_nentries], Bz_coil[MAX_nentries];
  
  Double_t err1[MAX_nentries];
  
  //

  ifstream txtfile1;
   
  char file1[200];
  sprintf(file1,"std_puck_air2.dat");
  // x y zc Bz_coil
  // Jay Opera model of magnetized beam gun magnet
  //
  cout << "Data file is " << file1 << endl;
  
  txtfile1.open(file1);
  
  UInt_t n1=0;
  while(1) {   
    txtfile1 >> x[n1] >> y[n1] >> zc[n1] >> Bz_coil[n1];
    err1[n1] = 0.0;
    
    if(!txtfile1.good()) break;
    n1++;
  }
  
  //
  
  for(Int_t i=0; i<n1; i++){
    
    Bz_coil[i] = -1.0*Bz_coil[i];
    zc[i]      =  zc[i] - 6.0; 
  }

  /////

  ifstream txtfile2;
   
  char file2[200];
  sprintf(file2,"std_puck_steel2.dat");
  // x y zc Bz_coil_puck
  // Jay Opera model of magnetized beam gun magnet
  //
  cout << "Data file is " << file2 << endl;
  
  txtfile2.open(file2);
  
  UInt_t n2=0;
  while(1) {   
    txtfile2 >> x[n2] >> y[n2] >> zp[n2] >> Bz_coil_puck[n2];
    err1[n2] = 0.0;
    
    if(!txtfile2.good()) break;
    n2++;
  }
  
  //
  
  for(Int_t i=0; i<n2; i++){
    
    Bz_coil_puck[i] = -1.0*Bz_coil_puck[i];
    zp[i]           = zp[i] - 6.0; 
  }
  
  //////
  
  b1 = new TCanvas("b1","Gun Magnet Field",40,40,700,500);
  b1->SetGrid();
  
  TGraphErrors *grS1 = new TGraphErrors(n1,zc,Bz_coil,err1,err1);
  TGraphErrors *grS2 = new TGraphErrors(n2,zp,Bz_coil_puck,err1,err1);
  
  grS1->GetXaxis()->SetLimits(0.0,100.0);
  grS1->SetMaximum(4000.0);
  grS1->SetMinimum(0.0);
  
  grS1->SetMarkerColor(kBlue);
  grS1->SetLineColor(kBlue);
  grS1->SetLineWidth(4);
  grS1->SetMarkerSize(1.0);
  grS1->SetMarkerStyle(20);
  grS1->SetLineStyle(1);
  
  grS1->GetXaxis()->CenterTitle();
  grS1->GetXaxis()->SetTitleOffset(1.2);
  grS1->GetXaxis()->SetTitleSize(0.06);
  TString tit = "z (cm)";
  grS1->GetXaxis()->SetTitle(tit.Data());
  
  grS1->GetYaxis()->CenterTitle();
  grS1->GetYaxis()->SetTitleOffset(1.0);
  grS1->GetYaxis()->SetTitleSize(0.06);
  TString tit = "B_{z} (Gauss)";
  grS1->GetYaxis()->SetTitle(tit.Data());
  
  grS1->Draw("A L");
  
  //
  
  grS2->SetLineColor(kRed);
  grS2->SetLineWidth(4);
  grS2->SetLineStyle(7);
  grS2->Draw("same L");
  
  //
  
  leg = new TLegend(0.45,0.70,0.85,0.85);
  leg->SetFillColor(0);
  leg->SetShadowColor(0);
  leg->AddEntry(grS1,"Cathode Solenoid","L");
  leg->AddEntry(grS2,"Cathode Solenoid + Hybrid Puck","L");
  leg->Draw();
  leg->Draw();
  
  // //
  
  b1->cd();
  pstit = "GunMagnet_Bz.gif";
  b1->Print(pstit.Data());
  
}