void exampleChisquare()  {

   RooWorkspace w;
   w.factory("Gaussian::g(x[-3,3],m[0,-3,3],s[1,0,10])");

   auto g = w.pdf("g");
   auto x = w.var("x");
   x->setBins(50);
   RooArgSet pars(*w.var("m"), *w.var("s")); 
   RooArgSet obs(*x);
   
   auto data = g->generate(obs, 5000);  
   g->fitTo(*data);

   auto pl = x->frame();
   data->plotOn(pl);
   g->plotOn(pl);

   auto c = new TCanvas();
   c->Divide(2,1);
   c->cd(1); 
   pl->Draw("SAME"); 

   auto h1 = data->createHistogram("x");
   auto f1 = g->asTF(obs, RooArgSet(), obs);
   auto fcor = new TF1("f2",[&](double* x, double *p){return f1->EvalPar(x,p)*h1->GetSumOfWeights()*h1->GetBinWidth(1); },-3,3,0);
   // clone the function to keep it alive in the plot
   auto f2 = (TF1*) fcor->Clone(); 

   c->cd(2);
   h1->Draw();
   f2->Draw("SAME"); 

   double chi2  = h1->Chisquare(fcor,"L");
   cout << "chi2 = " << chi2 << " chi2/ndf = " << chi2/(h1->GetNbinsX()-2) << std::endl;

   cout << "chi2/ndf from RooPlot = " << pl->chiSquare() << std::endl;
   
}