void graph2dminmax()
{
   TCanvas *c = new TCanvas("c","Graph2D example",0,0,600,600);
   Double_t Px = 6.;
   Double_t Py = 6.;
   Int_t np   = 1000;    // generate this many nodes

   Double_t *rx=0, *ry=0, *rz=0;
   rx = new Double_t[np];
   ry = new Double_t[np];
   rz = new Double_t[np];

   TRandom *r = new TRandom();
   
   for (Int_t N=0; N<np; N++) {
      rx[N]=2*Px*(r->Rndm(N))-Px;
      ry[N]=2*Py*(r->Rndm(N))-Py;
      rz[N]=sin(sqrt(rx[N]*rx[N]+ry[N]*ry[N]))+1;
   }

   TGraph2D *dt = new TGraph2D( np, rx, ry, rz);
   dt->Draw("P0");
   dt->GetXaxis()->SetLimits(-10,10);
   dt->GetYaxis()->SetLimits(-10,10);
   dt->SetMaximum(10);
   dt->SetMinimum(-10);
}