Double_t myFun (Double_t *xarg, Double_t *par) 
{
  Double_t x = xarg[0] , result = 0.; 

  for (int st=0; st<=3; st++) 
    result += par[st] * TMath::Power (x, st);

  return result;
}


void macro_FitTGraphErrors () {

 TCanvas* c1 = new TCanvas ("c1");
 
 TGraphErrors* gr = new TGraphErrors ("dataPoints.txt");
 gr->SetTitle ();
 gr->Draw("AP");

 TF1* fun = new TF1 ("fun", myFun , -3, 5, 4);
 fun->SetParameters (-1. , 1. , -6. , 1.);
 fun->SetParLimits (0, -3., 2.);
 gr->Fit ( fun , "" );

 c1->Update();
// cin.ignore();
}