void ReverseXAxis(TH1 *h);
void ReverseYAxis(TH1 *h);

void hist021_TH2_reverse_axis()
{
   TH2F *hpxpy = new TH2F("hpxpy", "py vs px", 40, -4, 4, 40, -4, 4);
   Float_t px, py;
   TRandom r;
   for (Int_t i = 0; i < 25000; i++) {
      r.Rannor(px, py);
      hpxpy->Fill(px, py);
   }
   TCanvas *c1 = new TCanvas("c1");
   hpxpy->Draw("colz");
   ReverseXAxis(hpxpy);
   ReverseYAxis(hpxpy);
}

void ReverseXAxis(TH1 *h)
{
   // Remove the current axis
   h->GetXaxis()->SetLabelOffset(999);
   h->GetXaxis()->SetTickLength(0);

   // Redraw the new axis
   gPad->Update();
   TGaxis *newaxis = new TGaxis(gPad->GetUxmax(), gPad->GetUymin(), gPad->GetUxmin(), gPad->GetUymin(),
                                h->GetXaxis()->GetXmin(), h->GetXaxis()->GetXmax(), 510, "-");
   newaxis->SetLabelOffset(-0.03);
   newaxis->Draw();
}

void ReverseYAxis(TH1 *h)
{
   // Remove the current axis
   h->GetYaxis()->SetLabelOffset(999);
   h->GetYaxis()->SetTickLength(0);

   // Redraw the new axis
   gPad->Update();
   TGaxis *newaxis = new TGaxis(gPad->GetUxmin(), gPad->GetUymax(), gPad->GetUxmin() - 0.001, gPad->GetUymin(),
                                h->GetYaxis()->GetXmin(), h->GetYaxis()->GetXmax(), 510, "+");
   newaxis->SetLabelOffset(-0.03);
   newaxis->Draw();
}