TH2F *h2;
TH1D *projh2X;
TH1D *projh2Y;
TPad *right_pad, *top_pad;

void hist019_TH2_projection()
{
   auto c1 = new TCanvas("c1", "c1", 900, 900);
   gStyle->SetOptStat(0);

   TPad *center_pad = new TPad("center_pad", "center_pad", 0.0, 0.0, 0.6, 0.6);
   center_pad->Draw();

   right_pad = new TPad("right_pad", "right_pad", 0.55, 0.0, 1.0, 0.6);
   right_pad->Draw();

   top_pad = new TPad("top_pad", "top_pad", 0.0, 0.55, 0.6, 1.0);
   top_pad->Draw();

   h2 = new TH2F("h2", "", 40, -4, 4, 40, -20, 20);
   Float_t px, py;
   for (Int_t i = 0; i < 25000; i++) {
      gRandom->Rannor(px, py);
      h2->Fill(px, 5 * py);
   }
   projh2X = h2->ProjectionX();
   projh2Y = h2->ProjectionY();

   center_pad->cd();
   gStyle->SetPalette(1);
   h2->Draw("COL");

   top_pad->cd();
   projh2X->SetFillColor(kBlue + 1);
   projh2X->Draw("bar");

   right_pad->cd();
   projh2Y->SetFillColor(kBlue - 2);
   projh2Y->Draw("hbar");

   c1->cd();
   TLatex t;
   t.SetTextFont(42);
   t.SetTextSize(0.02);
   t.DrawLatex(0.6, 0.88, "This example demonstrates how to display");
   t.DrawLatex(0.6, 0.85, "a histogram and its two projections.");

   auto ex = new TExec("zoom", "ZoomExec()");
   h2->GetListOfFunctions()->Add(ex);
}

void ZoomExec()
{
   int xfirst = h2->GetXaxis()->GetFirst();
   int xlast = h2->GetXaxis()->GetLast();
   double xmin = h2->GetXaxis()->GetBinLowEdge(xfirst);
   double xmax = h2->GetXaxis()->GetBinUpEdge(xlast);
   projh2X->GetXaxis()->SetRangeUser(xmin, xmax);
   top_pad->Modified();

   int yfirst = h2->GetYaxis()->GetFirst();
   int ylast = h2->GetYaxis()->GetLast();
   double ymin = h2->GetYaxis()->GetBinLowEdge(yfirst);
   double ymax = h2->GetYaxis()->GetBinUpEdge(ylast);
   projh2Y->GetXaxis()->SetRangeUser(ymin, ymax);
   right_pad->Modified();
}