{

   TFile f("hsimple.root");
   auto t  = (TTree*) f.Get("ntuple"); 
   t->Draw("px-py","");
   double * vz = ntuple->GetV1();
   RooRealVar z("z","z",-10,10);

   RooDataSet d("data","data",z);
   for (int i = 0; i < t->GetEntries(); ++i) {
      z.setVal(vz[i]);
      d.add(z);
   }

   RooPlot * pl = z.frame();
   d.plotOn(pl);
   pl->Draw();
}