void chi2_fit()
{
  TH1* h = new TH1F( "h", "#chi^{2}-Anpassung", 5, 0, 5 );
  h->SetStats( kFALSE );
  gStyle->SetOptFit(1111);
  h->SetMaximum( 3 );
  h->Draw();

  const UInt_t N = 3;
  double x[N] = { 1, 3, 4 };
  double y[N] = { 0.8, 1.8, 2.2 };
  //double e[N] = { 0.1, 0.2, 0.1 };
  // double e[N] = { 0.04, 0.06, 0.02 };
  //double e[N] = { 0.0, 0.0, 0.0 };
  double e[N] = { 1.0, 1.0, 1.0 };

  TGraphErrors* g = new TGraphErrors( N, x, y, 0, e );
  g->SetMarkerStyle( 20 );
  g->SetMarkerSize( 1 );
  g->Draw( "p" );

  TF1* f1 = new TF1( "f1", "pol1", 0, 5 );
  TF1* f2 = new TF1( "f2", "[0]+[1]*x", 0, 5 );
  TF1* f3 = new TF1( "f3", straightline, 0, 5, 2 );

  g->Fit( "pol1" );
  g->Fit( "f1" );
  g->Fit( "f2" );
  g->Fit( "f3" );
}

double straightline( double* x, double* par )
{
  return par[0] + par[1]*x[0];
}