// Reads the points from a file and produces a simple graph.
int read_graph_txt(){
    auto c=new TCanvas();
    //c->SetGrid();

    
    // Read data from txt file TGraphErrors
    TGraphErrors graph1("./mean_variation.txt","%lg %lg %lg %lg");     // * means not select this column of data
    graph1.SetTitle("Mean Variation_data;" 
                    "#gamma Energy (GeV);"
                    "#mu of pi0_M_Prefit");
   
    graph1.SetLineWidth(2);
    graph1.SetLineColor(4);
    graph1.SetMarkerStyle(20);
    graph1.SetMarkerColor(kBlue);
    graph1.SetDrawOption("P");
    // graph.GetXaxis()->SetRangeUser(0.0, 0.015); 
    // graph.GetYaxis()->SetRangeUser(0.12, 0.15); 
    graph1.GetXaxis()->CenterTitle();
    graph1.GetYaxis()->CenterTitle();
    graph1.GetXaxis()->SetTitleSize(0.04);
    graph1.GetYaxis()->SetTitleSize(0.04);
    // graph1.DrawClone("AP");
   

    // Print the data pount
    graph1.Print();
    cout<< "================================================"<< endl;

   

   // Read MC from txt file TGraphErrors
    TGraphErrors graph2("./mean_variation_new_single_gauss.txt","%lg %lg %lg %lg");     // * means not select this column of data
    graph2.SetTitle("Mean Variation_MC;" 
                    "#gamma Energy (GeV);"
                    "#mu of pi0_M_Prefit");
   
    graph2.SetLineWidth(2);
    graph2.SetLineColor(4);
    graph2.SetMarkerStyle(20);
    graph2.SetMarkerColor(kRed);
    // graph2.SetDrawOption("P");
    // graph.GetXaxis()->SetRangeUser(0.0, 0.015); 
    // graph.GetYaxis()->SetRangeUser(0.12, 0.15); 
    graph2.GetXaxis()->CenterTitle();
    graph2.GetYaxis()->CenterTitle();
    graph2.GetXaxis()->SetTitleSize(0.04);
    graph2.GetYaxis()->SetTitleSize(0.04);
    // graph2.DrawClone("AP");
   

    // Print the data pount
    graph2.Print();
    cout<< "================================================"<< endl;
    
    

    // MultiGraph
    // TMultiGraph  *mg  = new TMultiGraph();
    TMultiGraph (mg);
    mg.Add(graph1);  //, "L"
    mg.Add(graph2);
    mg.Draw("APL");

    c->Update();
    c->GetFrame()->SetBorderSize(12);
    c->Modified();
    // c->SetLogy();
    // c->SetLogx();
   
     return 0;
}