void graph2d20()
{

   TCanvas *c1 = new TCanvas("c1","Graph2D example",0,0,700,600);

   Double_t P = 5.;
   Int_t npx  = 20 ;
   Int_t npy  = 20 ;

   Double_t x = -P;
   Double_t y = -P;
   Double_t z;
   Int_t k = 0;

   Double_t dx = (2*P)/npx;
   Double_t dy = (2*P)/npy;

   TGraph2D *dt = new TGraph2D(npx*npy);
   dt->SetNpy(41);
   dt->SetNpx(40);

   for (Int_t i=0; i<npx; i++) {
      for (Int_t j=0; j<npy; j++) {
         z = sin(sqrt(x*x+y*y))+1;
         dt->SetPoint(k,x,y,z);
         k++;
         y = y+dx;
      }
      x = x+dx;
      y = -P;
   }

   gStyle->SetOptStat(0);
   gStyle->SetPalette(kBlueRedYellow );
   gStyle->SetNumberContours(90);

   dt->GetXaxis()->SetTitle("X title");
   dt->GetYaxis()->SetTitle("Y title");

   dt->GetXaxis()->SetLabelFont(42);
   dt->GetYaxis()->SetLabelFont(42);
   dt->GetZaxis()->SetLabelFont(42);

   dt->GetXaxis()->SetTitleFont(42);
   dt->GetYaxis()->SetTitleFont(42);

   dt->Draw("SURF1");
}