reversegraphaxis ()
{
   const Int_t n = 20;
   Double_t x[n], y[n];
   for (Int_t i=0;i<n;i++) {
      x[i] = i*0.1+5;
      y[i] = 10*sin(x[i]+0.2);
   }
   g = new TGraph(n,x,y);
   g->SetMarkerStyle(21);
   g->Draw("APL");

   ReverseYAxis(g);
   ReverseYGraph(g);
}

ReverseYAxis (TGraph *g)
{
   // Remove the current axis
   g->GetYaxis()->SetLabelOffset(999);
   g->GetYaxis()->SetTickLength(0);

   // Redraw the new axis 
   gPad->Update();
   TGaxis *newaxis = new TGaxis(gPad->GetUxmin(), 
                                gPad->GetUymax(),
                                gPad->GetUxmin()-0.001,
                                gPad->GetUymin(),
                                g->GetYaxis()->GetXmin(),
                                g->GetYaxis()->GetXmax(),
                                510,"+");
   newaxis->SetLabelOffset(-0.04);
   newaxis->Draw();
}

ReverseYGraph (TGraph *g)
{
   // Create a new graph
   Int_t n = g->GetN();
   Double_t *x = g->GetX();
   Double_t *y = g->GetY();
   Double_t yr[100];
   Double_t dy = g->GetYaxis()->GetXmin()+g->GetYaxis()->GetXmax();
   for (Int_t i=0; i<n; i++) {
      yr[i] = -y[i]+dy;
   }
   
   gr = new TGraph(n,x,yr);
   gr->SetMarkerStyle(20);
   gr->SetLineColor(kRed);
   gr->SetMarkerColor(kRed);
   gr->Draw("PL");
}