The mean values for the pixel 255_255 is stored in a text file and a linear fit is perforemd

linear_fit.cpp

+void gain_4()
+{
+  auto *injected_data = new TGraph("mean_values.txt");
+  auto *noise_data = new TGraph("noise_mean.txt");
+
+  injected_data->SetMarkerColor(4);
+  injected_data->SetMarkerStyle(20);
+
+
+  // Define Linear fit function
+
+  auto linear_fit = new TF1("linear_fit","[0]*x +[1]",0,300);
+  linear_fit->SetParNames("slope","offset");
+  linear_fit->SetParameter(0,2);
+  linear_fit->SetParameter(1,1200);
+  linear_fit->SetLineColor(2);
+
+  injected_data->Fit(linear_fit);
+  // gr->Draw("AP");
+
+  noise_data->SetMarkerColor(6);
+  noise_data->SetMarkerStyle(21);
+
+  TMultiGraph *g = new TMultiGraph();
+  g->Add(injected_data);
+  g->Add(noise_data);
+  g->Draw("AP");
+  g->GetXaxis()->SetTitle("Injected voltage");
+  g->GetYaxis()->SetTitle("Threshold");
+  
+ 
+
+   
+
+   TLegend *leg = new TLegend(0.2,0.2,0.4,0.4);
+   leg->SetBorderSize(0);
+   leg->AddEntry(injected_data,"Injected data","p");
+   leg->AddEntry(linear_fit,"linear fit","l");
+   leg->AddEntry(noise_data,"noise","p");
+  
+   
+   gStyle->SetOptFit();
+   // gStyle->SetBorderSize(0);
+	   
+   leg->Draw();
+
+  
+
+}