void treer() {
   TFile f("junk.root");
   TCanvas c1;
   Int_t first = 0;
   while(1) {
      f.ReadKeys();
      TTree *ntuple = (TTree*)f.Get("ntuple");
      if (first == 0) ntuple->Draw("px>>hpx","","",10000000,first);
      else            ntuple->Draw("px>>+hpx","","",10000000,first);
      first = (Int_t)ntuple->GetEntries();
      delete ntuple;
      c1.Update();
      gSystem->Sleep(1000); //sleep 1 second
   }
}