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_FitErrorMatrix () {

 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.);

 TFitResultPtr fitRes = gr->Fit ( fun , "S" );
 TMatrixDSym cov = fitRes->GetCovarianceMatrix ();

 for (int r = 0; r < cov.GetNrows() ; r++) {
  for (int c = 0; c < cov.GetNcols() ; c++)
   cout << setw(16) << cov[r][c] ;

  cout << endl;
 }

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