Problem with tree and histogram

ROOT
12

A brutal fix:

{
  gStyle->SetPalette(55);
  TFile *f = TFile::Open("Maxwell.root");
  TTree *t; f->GetObject("tree", t);
  TH3F *h3 = new TH3F("h3", "Representation of gas in the sphere", 10, -1, 1, 10, -1, 1, 10, -1, 1);
  t->Project("h3", "z:y:x", "sqrt(x*x+y*y+z*z)>0", "", 100000);
  ((TArrayF *)h3)->Reset(); // "reset" histogram's bins' contents
  t->Draw("z:y:x:8*sqrt(x*x+y*y+z*z)>>+h3", "sqrt(x*x+y*y+z*z)>0", "colz", 100000);
}

BTW. Another example can be found in this thread. For "e1:e2" draw expressions, which produce unbinned 2-d scatter-plots (TGraph), there is this thread.