void fit_dr(void)
{
// 
// plot the number of fits to a cdc segment that result in prob > prob_cut (currently set to 0.01).
//

#include <TMath.h>
#include <TRandom.h>

gROOT->Reset();
//TTree *Bfield = (TTree *) gROOT->FindObject("Bfield");
gStyle->SetPalette(1,0);
gStyle->SetOptStat(kFALSE);
gStyle->SetOptFit(kFALSE);
// gStyle->SetOptFit(1111);
gStyle->SetPadRightMargin(0.15);
gStyle->SetPadLeftMargin(0.15);
gStyle->SetPadBottomMargin(0.15);
gStyle->SetFillColor(0);
//

   char string[256];
   Int_t j,jj;
   
   #define npts 6;

   // input data set first point (actually -1) to -10 so that it is not plotted.

   // use offset on x-axis to make data visible and give negative y-values for reference (not plotted)

   Double_t temp[npts]={-60,-53,-22,0,25,40};
   Double_t xerr[npts]={0.1,0.1,0.1,0.1,0.1};

   // Dark rates vs temperature at fixed overvoltage.

   Double_t dr2V[npts]={0,800,4000,30000,400000,0};
   Double_t dr3V[npts]={-1,1500,7000,50000,650000,-1};
   Double_t dr_err[npts]={1,1,1,1,1,1};

   Double_t xmin=-60;
   Double_t xmax=40;
   Double_t ymin=100;
   Double_t ymax=5e6;

   //
   TCanvas *c1 = new TCanvas("c1","Dark rates vs temperature at fixed overvoltage",200,10,700,700);
   c1->SetBorderMode(0);
   c1->SetFillColor(0);

   c1->SetGridx();
   c1->SetLogy();
   c1->SetGridy();
   c1->SetBorderMode(0);
   c1->SetFillColor(0);


   TGraph *dr2 = new TGraph (npts,temp,dr2V);
   TGraph *dr3 = new TGraph (npts,temp,dr3V);

   TLegend *leg = new TLegend(0.15,0.8,0.75,0.9);

   dr2->SetTitle("");
   dr2->GetXaxis()->SetRangeUser(xmin,xmax);
   dr2->GetYaxis()->SetRangeUser(ymin,ymax);
   dr2->GetXaxis()->SetTitleSize(0.04);
   dr2->GetYaxis()->SetTitleSize(0.04);
   dr2->GetYaxis()->SetTitleOffset(1.5);
   dr2->GetXaxis()->SetTitle("Temperature (degC)");
   dr2->GetYaxis()->SetTitle("Dark Rate (Hz)");
   dr2->GetXaxis()->SetNdivisions(6);

   dr2->SetMarkerColor(2);
   dr2->SetMarkerStyle(20);
   dr2->Draw("AP");

   dr3->SetMarkerColor(4);
   dr3->SetMarkerStyle(20);
   dr3->Draw("P");

   Double_t xminfit = -40;
   Double_t xmaxfit = 27;
   Double_t T0=20;
   Double_t a=500000;
   Double_t b=0.04;
   // Double_t c=-0.001;
   Int_t npar=2;
   TF1 *exp1 = new TF1("exp1",exp1,xminfit-T0,xmaxfit,npar);
   // exp1->SetParameters(a,b,c);
   exp1->SetParameters(a,b);
   exp1->SetLineColor(2);
   dr2->Fit("exp1","","",xminfit,xmaxfit);
   exp1->Draw("same");
   a = exp1->GetParameter(0);
   b = exp1->GetParameter(1);
   // c = exp1->GetParameter(2);
   // sprintf (string,"#Delta V=2V, a=%.0f, b=%.3f, c=%.5f\n",a,b,c);
   sprintf (string,"#Delta V=2V, a=%.0f, b=%.3f\n",a,b);
   leg->AddEntry(dr2,string,"p");
   printf ("string=%s\n",string);
   t1 = new TLatex(0.2,0.7,string);
   t1->SetTextColor(1);
   t1->SetNDC();
   t1->SetTextSize(0.03);
   // t1->Draw();


   Double_t a=500000;
   Double_t b=0.04;
   Double_t c=-0.001;
   Int_t npar=3;
   exp1->SetParameters(a,b,c);
   exp1->SetLineColor(4);
   dr3->Fit("exp1","","",xminfit,xmaxfit);
   exp1->DrawCopy("same");
   a = exp1->GetParameter(0);
   b = exp1->GetParameter(1);
   // c = exp1->GetParameter(2);
   // sprintf (string,"#Delta V=3V, a=%.0f, b=%.3f, c=%.5f\n",a,b,c);
   sprintf (string,"#Delta V=3V, a=%.0f, b=%.3f\n",a,b);
   leg->AddEntry(dr3,string,"p");
   printf ("string=%s\n",string);
   t1 = new TLatex(0.2,0.65,string);
   t1->SetTextColor(1);
   t1->SetNDC();
   t1->SetTextSize(0.03);
   // t1->Draw();

   leg->Draw();


   //

   c1->SaveAs("fit_dr.eps");
   c1->SaveAs("fit_dr.gif");

}
Double_t exp1 (Double_t *x, Double_t *par) {

   Double_t a=par[0];
   Double_t b=par[1];
   // Double_t c=par[2];
   Double_t c=0;
   Double_t T0=20;
   Double_t x1=x[0]-T0;

   char string[256];

   Double_t exp1 = a*exp(b*x1 + c*x1*x1);
   
   /*sprintf (string,"exp1=%f a=%f b=%f c=%f\n",exp1,a,b,c);
   printf ("string=%s",string);*/

   return exp1;
}