th2polyUSA.C: This tutorial illustrates how to create an histogram with polygonal

//This tutorial illustrates how to create an histogram with polygonal
//bins (TH2Poly), fill it and draw it using GL. The initial data are stored
//in TMultiGraphs. They represent the USA.
//
//The initial data have been downloaded from: http://www.maproom.psu.edu/dcw/
//This database was developed in 1991/1992 and national boundaries reflect
//political reality as of that time.
//
//Author: Olivier Couet

void th2polyUSA()
{
   Int_t i, bin;
   const Int_t nx = 48;
   char *states [nx] = {
      "alabama",      "arizona",        "arkansas",       "california",
      "colorado",     "connecticut",    "delaware",       "florida",
      "georgia",      "idaho",          "illinois",       "indiana",
      "iowa",         "kansas",         "kentucky",       "louisiana",
      "maine",        "maryland",       "massachusetts",  "michigan",
      "minnesota",    "mississippi",    "missouri",       "montana",
      "nebraska",     "nevada",         "new_hampshire",  "new_jersey",
      "new_mexico",   "new_york",       "north_carolina", "north_dakota",
      "ohio",         "oklahoma",       "oregon",         "pennsylvania",
      "rhode_island", "south_carolina", "south_dakota",   "tennessee",
      "texas",        "utah",           "vermont",        "virginia",
      "washington",   "west_virginia",  "wisconsin",      "wyoming"
   };
   Float_t pop[nx] = {
    4708708, 6595778,  2889450, 36961664, 5024748,  3518288,  885122, 18537969,
    9829211, 1545801, 12910409,  6423113, 3007856,  2818747, 4314113,  4492076,
    1318301, 5699478,  6593587,  9969727, 5266214,  2951996, 5987580,   974989,
    1796619, 2643085,  1324575,  8707739, 2009671, 19541453, 9380884,   646844,
   11542645, 3687050,  3825657, 12604767, 1053209,  4561242,  812383,  6296254,
   24782302, 2784572,   621760,  7882590, 6664195,  1819777, 5654774,   544270
   };

   gStyle->SetCanvasPreferGL(true);
   TCanvas *usa = new TCanvas("USA", "USA");
   usa->ToggleEventStatus();
   Double_t lon1 = -130;
   Double_t lon2 = -65;
   Double_t lat1 = 24;
   Double_t lat2 = 50;
   TH2Poly *p = new TH2Poly("USA","USA Population",lon1,lon2,lat1,lat2);

   TFile *f;
   f = TFile::Open("http://root.cern.ch/files/usa.root");

   // Define the TH2Poly bins.
   TMultiGraph *mg;
   TKey *key;
   TIter nextkey(gDirectory->GetListOfKeys());
   while (key = (TKey*)nextkey()) {
      obj = key->ReadObj();
      if (obj->InheritsFrom("TMultiGraph")) {
         mg = (TMultiGraph*)obj;
         bin = p->AddBin(mg);
      }
   }

   // Fill TH2Poly.
   for (i=0; i<nx; i++) p->Fill(states[i], pop[i]);

   gStyle->SetOptStat(11);
   gStyle->SetPalette(1);
   p->Draw("gllego");
}