void TGraphFit() {
   //Draw a graph with error bars and fit a function to it.
   //Originial source Rene Brun    modified: Gunter Quast
   
  //set global options
  gStyle->SetOptFit(111);  //superimpose fit results

  // make nice Canvas 
  TCanvas *c1 = new TCanvas("c1","Daten",200,10,700,500);
  c1->SetGrid();

  //define some data points 
  const Int_t n = 10;
  Float_t x[n]  = {-0.22, 0.1, 0.25, 0.35, 0.5, 0.61, 0.7, 0.85, 0.89, 1.1};
  Float_t y[n]  = {0.7, 2.9, 5.6, 7.4, 9., 9.6, 8.7, 6.3, 4.5, 1.1};
  Float_t ey[n] = {.8,.7,.6,.5,.4,.4,.5,.6,.7,.8};
  Float_t ex[n] = {.05,.1,.07,.07,.04,.05,.06,.07,.08,.05};
  //  Float_t ex[n] = {.0,.0,.0,.0,.0,.0,.0,.0,.0,.0};  for test

  // copy data to TGraphErros object
  TGraphErrors *gr = new TGraphErrors(n,x,y,ex,ey);
  gr->SetTitle("TGraphErrors mit Fit");
  gr->Draw("AP");

  // now perform a fit (with errors in x and y!)
   gr->Fit("gaus");

   c1->Update();
}