#include "Riostream.h" 

void fpol2()   { 
   //THANK YOU TO @DABAGIAN 
   //MODIFY JULY 8/2004 BY @KRISFRAJER 

   const Int_t n = 5; 

   char fname[100], message[50]; 
   ifstream in; 
   Int_t ring=2; 
   Int_t run=3173; 
   Int_t chann=21; 
   Double_t p, q; 
   Double_t x[n], y[n]; 
   TF1 *myfit; 

   sprintf(fname,"fpol2.dat"); 
   in.open(fname); 

   c1 = new TCanvas("c1","Transmission",20,50,1200,800); 

   c1->SetFillColor(kWhite); 
   c1->SetGrid(); 
   //line26 
   myfit = new TF1("myfit","[0]+[1]*x", 3170, 3181); 

   //I was trying to get a square fit, so a replace the //formula on myfit object to something
   // like "pol2" //and "[0]+[1]*x+[2]*x^2". None of this work. What 
   //was I doing wrong? 

   for (Int_t i=0;i<n;i++){ 
   in >> p >> q; 
   x[i] = p; 
   y[i] = q; 
   printf(" i %i %f %f \n",i,x[i],y[i]); 
   } 

   gr = new TGraph(n,x,y); 
   gr->SetLineColor(1); 
   gr->SetLineWidth(1); 
   gr->SetMarkerColor(4); 
   gr->SetMarkerStyle(25); 
   sprintf(message,"detector %d",chann); 

   //line46 
   gr->Fit("myfit","R"); 
   //I found out that if I write the last line as 
   //gr->Fit("pol2"); 
   //my program worked! 
   myfit->Draw();
   myfit->SetTitle(message); 
   myfit->GetXaxis()->SetTitle("runs"); 
   myfit->GetYaxis()->SetTitle("Signal Output"); 
   gr->Draw("CP"); 

   // TCanvas::Update() draws the frame, after 
   // which one can change it 
   c1->Update(); 
   c1->GetFrame()->SetFillColor(kWhite); 
   c1->GetFrame()->SetBorderSize(2); 
   c1->Modified(); 
   in.close(); 
}