Double_t fitfunc(Double_t *x, Double_t *par)
{
  Double_t fitval = par[0]+x[0]*par[1]+x[0]*x[0]*par[2];
  return fitval;
}
void danilo2()
  {
     TFile *f = new TFile("daten.root");
     TGraphErrors *data = (TGraphErrors*)f->Get("data");
     TF1 dummy("dummy","gaus",0,1); //keep this
     TF1 *function = new TF1("fitfunc",fitfunc,0,1,3);
     function->SetParNames("const","linear","quadratic");
     function->SetParameters(18,14,-9);
     data->Draw("ap");
     data->Fit("fitfunc");
  }