void drawSAME()
{
   TTree *t = new TTree("Tthree","three variables");

   Double_t x, y, z;
   Double_t xmin=0;
   Double_t xmax=0;

   t->Branch("X",&x,"X/D");
   t->Branch("Y",&y,"Y/D");
   t->Branch("Z",&z,"Z/D");

   for (int i=0; i<1000; i++) {
      gRandom->Rannor(x,y);
      z = x + y;
      t->Fill();
      if (x<xmin) xmin=x;
      if (x>xmax) xmax=x;
   }

   TCanvas *c = new TCanvas("c","see");

   TF1 *diag = new TF1("diag", "x", xmin, xmax);
   TGraph *diaGraph = new TGraph(diag);

   t->Draw("X:Y:Z", "", "COL");

   diaGraph->Draw("L");
}