void chi2_prob()
{
  TH1* h = new TH1F( "h", "#chi^{2}-Wahrscheinlichkeit", 10, 0, 25 );
  h->SetStats( kFALSE );
  h->SetXTitle( "#chi^{2}" );
  h->SetYTitle( "P_{#chi^{2}}" );
  h->Draw();


  TF1* f = new TF1("f", prob, 0, 25, 1 );

  for( int i = 2; i <= 10; i +=2 ) {
    f->SetParameter( 0, i );
    f->SetLineColor( i+1 );
    f->DrawCopy( "same" );
  } 
}

double prob( double* x, double* par )
{
  return TMath::Prob( x[0], par[0] );
}