void gerrors()
{
   c1 = new TCanvas("c1","A Simple Graph with error bars",200,10,700,500);
   c1->SetGrid();
   c1->SetLogx();
   c1->SetLogy();
   Double_t x[13]  = {1,3,5,7,10,30,50,70,100,300,500,700,1000};
   Double_t y[13]  = {4.62,44.5,115,205,400,2413,5339,9097,15515,73328,143943,221128,337753};
   DrawGraphArrow(13,x,y);
}

DrawGraphArrow(Int_t n,Double_t *x, Double_t *y)
{
   TGraph *g = new TGraph(n,x,y);
   g->SetTitle("Effective Area");
   g->SetMarkerColor(4);
   g->SetMarkerStyle(21);
   g->Draw("AP");

   TArrow *a;
   Double_t yl;
   for (Int_t i=0; i<n; i++) {
      yl = TMath::Log10(y[i])-0.5;
      yl = TMath::Power(10,yl);
      a = new TArrow(x[i],y[i],x[i],yl,0.02,">");
      a->Draw();
   }

   g->Draw("P");
}