void treegen()
{
  const unsigned nPts = 23735;
  std::ifstream in("3Dtest_large.txt");
  if(!in)
  {
    std::cout<<"Can not open 3Dtest_large.txt\n";
    return;
  }

  TFile *f = new TFile("tree.root","recreate");
  TTree *t1 = new TTree("t1","a simple Tree with simple variables");
  double x = 0., y = 0., z = 0.;
  double v4 = 1.;

  t1->Branch("x",&x,"x/D");
  t1->Branch("y",&y,"y/D");
  t1->Branch("z",&z,"z/D");
  t1->Branch("v4",&v4,"v4/D");
  t1->Branch("dummy",&z,"dummy/D");

  for (unsigned i = 0; i < nPts; ++i) {
    in>>x>>y>>z;
    x = TMath::Log(x);
    y = TMath::Log(y);
    z = TMath::Log(z);
    t1->Fill();
  }

  t1->Draw("z:x:y:v4:dummy", "", "gl5d");
}