TH1F *DrawFrame(Double_t xmin, Double_t ymin, Double_t xmax, Double_t ymax, const char *title)
{
   //  Draw a pad frame
   // This function is a copy of TPad::DrawFrame in CVS head
   //
   //  Compute real pad range taking into account all margins
   //  Use services of TH1F class

   TH1F *hframe = (TH1F*)gPad->FindObject("hframe");
   if (hframe) delete hframe;
   Int_t nbins = 1000;
   //if log scale in X, use variable bin size linear with log(x)
   //this gives a better precision when zooming on the axis
   if (gPad->GetLogx() && xmin > 0 && xmax > xmin) {
      Double_t xminl = TMath::Log(xmin);
      Double_t xmaxl = TMath::Log(xmax);
      Double_t dx = (xmaxl-xminl)/nbins;
      Double_t *xbins = new Double_t[nbins+1];
      xbins[0] = xmin;
      for (Int_t i=1;i<=nbins;i++) {
         xbins[i] = TMath::Exp(xminl+i*dx);
      }
      hframe = new TH1F("hframe",title,nbins,xbins);
      delete [] xbins;
   } else {
      hframe = new TH1F("hframe",title,nbins,xmin,xmax);
   }
   hframe->SetBit(TH1::kNoStats);
   hframe->SetBit(kCanDelete);
   hframe->SetMinimum(ymin);
   hframe->SetMaximum(ymax);
   hframe->GetYaxis()->SetLimits(ymin,ymax);
   hframe->SetDirectory(0);
   hframe->Draw(" ");
   gPad->Update();
   return hframe;
}
void leggi()
{
  TCanvas *c1= new TCanvas("c1","c1",200,10,700,500);
  c1->SetLogy(kTRUE);
  c1->SetLogx(kTRUE);
  c1->SetGrid(kTRUE);
  int nlines=0,n=0;
  FILE *file;
  file=fopen("xs.dat","r");
  if(file==NULL)
    {
       cout << "Errore apertura file!" << endl;
    }

  char riga[10000];
  float energy_,xs_,xserr_;
  float energy[10000],xs[10000],xserr[10000];
  while(fscanf(file,"\n%[^\n]",riga)>EOF)
    {
       sscanf(riga, "%f %f %f", &energy_,&xs_,&xserr_);
       energy[n]=energy_;
       xs[n]=xs_;
       xserr[n]=xserr_;
       nlines++;
       n++;
    }
  fclose(file);
  cout << nlines << endl;

  TGraphErrors *gr = new TGraphErrors(nlines,energy,xs,0,xserr);
  gr->SetMarkerStyle(21);
  gr->SetMarkerSize(0.5);
  gr->SetMarkerColor(4);
  TH1F *hpx = DrawFrame(1e-2,1e-2,5e6,5e6,"Sezione d'urto^{245}Cm");
  hpx->GetXaxis()->CenterTitle();
  hpx->GetXaxis()->SetTitle("E_{n} (eV)");
  hpx->GetYaxis()->CenterTitle();
  hpx->GetYaxis()->SetTitle("#sigma (barn)");
  hpx->GetYaxis()->SetLimits(1e-2,5e6);
  gr->Draw("lp");
}