void Calibration(){
   auto fenetre = new TCanvas("Regression_lineaire", "Calibration", 700 , 500) ;
   fenetre->SetLeftMargin(0.15);
   gStyle->SetOptFit(1); // Display the fir parameters

   const Int_t n = 8 ;   // Total number of data points in file.

   // number of data in Si1 files : 11
   // number of data in Si2 files : 10
   // number of data in Si3 files : 8
   // number of data in Si4 files : 6

   //_____________________________________________________________________________________
   // Experimental datas.

   // Storing values of ENERGIES in a 1D array.
   Double_t E[n+3] ;
   Double_t C[n+3] ; // Uncertainty on the energy.

   Double_t X_a[3];
   Double_t Y_a[3];

   Double_t X[n];
   Double_t Y[n];

   // part for Si calibration
   ifstream data("E_loss_24_86_Si_3.txt"); // file storing channel datas.
   for(Int_t i = 0 ; i < n+3 ; i++){
          data >> C[i] >> E[i] ; // filling the arrays with the values of the file.
   }
   data.close();

   for (Int_t i = 0 ; i < n ; i++) {
      Y[i] = C[i];
      X[i] = E[i];
   }

   for (Int_t i = 0 ; i < 3 ; i++){
      Y_a[i] = C[n+i];
      X_a[i] = E[n+i];
   }


   TMultiGraph *mg = new TMultiGraph();
   mg->SetTitle("Calibration Ligne Faisceau");

   // Drawing the Graph with the TGraph class.
   TGraph *f1 = new TGraph(n, X, Y);
   f1->SetMarkerColor(4);
   f1->SetMarkerSize(2);
   f1->SetLineColor(4);

   TGraph *alpha = new TGraph(3, X_a, Y_a);
   alpha->SetMarkerColor(3);
   alpha->SetMarkerSize(2);
   alpha->SetLineColor(3);

   // adding all the Graphs into the total graph for the plot.
   mg->Add(f1);
   mg->Add(alpha);
   alpha->Draw("AL*");
   mg->Draw("AL*");

   mg->GetXaxis()->SetTitle("Energy [keV]");
   mg->GetYaxis()->SetTitle("Channels");

   //_____________________________________________________________________________________
   // Linear regression = linear fit with a straight line (type y = ax +b)

   TF1 *reglin = new TF1("Regression lineaire","[0]*x+[1]", 3000, 11000);
   reglin->SetParameters(0.2,2);
   reglin->SetParNames("Gain", "E0"); //parametrisation : a = gain ; b = offset energy (E0)

   mg->Fit(reglin);

   float chi2 = reglin->GetChisquare();
   cout << "chi2 = " << chi2 << endl;
}