void fit()
  
{
  TCanvas *c1 = new TCanvas("c1","c1",700,500);
  Int_t n = 20;

  // mu_Errm
  Double_t x[20] = {0.002723, 0.003313, 0.003625, 0.003854, 0.004033, 0.004178, 0.004296, 0.004406, 0.00451, 0.004618, 0.004739, 0.004873, 0.005035, 0.005227, 0.005451, 0.005723, 0.006051, 0.006504, 0.007195, 0.008657 };
  Double_t ex[20] = {7.95421e-06, 3.22325e-06, 2.15406e-06, 1.68393e-06, 1.34469e-06, 1.06632e-06, 9.06832e-07, 8.99227e-07, 8.64925e-07, 9.82474e-07, 1.02539e-06, 1.21688e-06, 1.44946e-06, 1.70204e-06, 2.06965e-06, 2.45717e-06, 3.07269e-06, 4.33906e-06, 7.71312e-06, 2.26849e-05};

 
  
  Double_t y[20] = {0.13327, 0.13322, 0.13272, 0.13311, 0.13293, 0.13309, 0.13317, 0.13351, 0.13400, 0.13389, 0.13408, 0.13392, 0.13408, 0.13403, 0.13376, 0.13387, 0.13390, 0.13415, 0.13414, 0.13502};
  Double_t ey[20] ={0.00016, 0.00015, 0.00016, 0.00016, 0.00016, 0.00016, 0.00014, 0.00016, 0.00016, 0.00015, 0.00016, 0.00016, 0.00017, 0.00017, 0.00018, 0.00019, 0.00019, 0.00020, 0.00023, 0.00023};
  



  // TGraph
  TGraph *gr1 = new TGraphErrors(n,x,y,ex,ey);
  gr1->SetMarkerStyle(20);
  gr1->SetMarkerColor(kBlue);
  gr1->SetDrawOption("P");
  gr1->SetLineColor(4);
  gr1->SetLineWidth(2);
  gr1->GetXaxis()->SetRangeUser(0,0.015); 
  gr1->GetYaxis()->SetRangeUser(0.130,0.137);  
  gr1->GetXaxis()->SetTitle("pi0_ErrM");
  gr1->GetYaxis()->SetTitle("#mu of pi0_M_prefit");
  gr1->GetXaxis()->CenterTitle();
  gr1->GetYaxis()->CenterTitle();
  gr1->GetXaxis()->SetTitleSize(0.05);
  gr1->GetYaxis()->SetTitleSize(0.04);


   
  
  // Range wise fit
  gr1->Sort(); // make sure points are in the "increasing order in x"
  Double_t par[7];

  // 0 to 0.004
  Double_t xmin1 = 0,  xmax1 = 0.0041;
  TF1 *f1 = new TF1("f1","pol1", xmin1, xmax1);

   // 0.004 to 0.0055
  Double_t xmin2 = 0.0041,  xmax2 = 0.0055;
  TF1 *f2 = new TF1("f2","pol2", xmin2, xmax2);

   // 0.0055 to 0.015
  Double_t xmin3 = 0.0055,  xmax3 = 0.015;
  TF1 *f3 = new TF1("f3","pol1", xmin3, xmax3);
  
  TF1 *total = new TF1("total","pol1(0)+pol2(2)+pol1(5)",0,0.015);

  gr1->Fit("f1", "", "", xmin1, xmax1);
  gr1->Fit("f2", "", "", xmin2, xmax2);
  gr1->Fit("f3", "", "", xmin3, xmax3);

  f1->GetParameters(&par[0]);
  f2->GetParameters(&par[2]);
  f3->GetParameters(&par[5]);
  total->SetParameters(par);
  gr1->Fit("total","", "", 0, 0.015);

  gr1->Draw("AP");


  
  c1->Update();
  //c1->GetFrame()->SetFillColor(21);
  c1->GetFrame()->SetBorderSize(12);
  c1->Modified();
   
  

 
}