#include "TH2.h"
#include "TRandom.h"
#include "TCanvas.h"
#include "TGaxis.h"

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

    void reverseaxis ()
    {
       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();
    }