auto myHist = new TH1F("h1","ntuple",100,-4,4);

auto myFile = TFile::Open("hsimple.root");
if (!myFile || myFile->IsZombie()) {
   return;
}

TTreeReader myReader("ntuple", myFile);

TTreeReaderValue<Float_t> myPx(myReader, "px");

TTreeReaderValue<Float_t> myPy(myReader, "py");

while (myReader.Next()) {
   // Just access the data as if myPx and myPy were iterators (note the '*'
   // in front of them):
   myHist->Fill(*myPx + *myPy);
}

myHist->Draw();

gROOT->GetListOfCanvases()->Draw()