Double_t fitf(Double_t *x, Double_t *par) 
{ 
  Double_t fitval(0); 
  
  if ( sqrt(x[0]*x[0]+x[1]*x[1])<90. ) 
  { 
    fitval=par[0]+par[1]*x[0]+par[2]*x[0]*x[0]+par[3]*x[1]+par[4]*x[1]*x[1]+par[5]*x[0]*x[1];    
  } else {fitval=0;} 
  return fitval; 
} 


void macro()
{
  gStyle->SetFrameFillColor(0);
  
  TCanvas *c1 = new TCanvas("c1","c1",600,500);
  gStyle->SetPalette(1);

  TFile *a = new TFile("graph.root");
  TGraph2D *g = (TGraph2D*)a->Get("Graph2D");

  TF2 *f2 = new TF2("f2",fitf,0.,90,0.,90.,6);
  f2->SetParameters(1.,1.,1.,1.,1.,1.);
  g->Fit("f2");
  
  f2->SetTitle("Minuit fit result on the Graph2D points");
  f2->GetXaxis()->SetTitle("X");
  f2->GetXaxis()->SetTitleOffset(2.0);
  f2->GetYaxis()->SetTitle("Y");
  f2->GetYaxis()->SetTitleOffset(2.0);
  f2->GetZaxis()->SetTitle("Z");
  f2->GetZaxis()->SetTitleOffset(1.2);
  f2->Draw("surf1 Z");
  g->Draw("same p0");
  
 gPad->SetRightMargin(0.15);

  gPad->SetTheta(40.2113);
  gPad->SetPhi(-54.79899);
}