void provaeffplateau()
{
   //
   // To see the output of this macro, click  here. 
   //
   gROOT->Reset();
   c1 = new TCanvas("c1","Efficiency plateau",200,10,700,500);

   c1->SetFillColor(0);
   c1->SetGrid();
   c1->GetFrame()->SetFillColor(21);
   c1->GetFrame()->SetBorderSize(12);

   //string fileName = "../data/thscan.dat";
   //char* fileName = "../data/hvscan.dat";
   char* fileName = "hvscan.dat";
   ifstream inputFile;
   //inputFile.open("../data/thscan.dat");
   inputFile.open(fileName);
   Int_t nP = 0;
   Float_t x[100];
   Float_t dx[100];
   Float_t eff_v[100];
   Float_t deff_v[100];
   while (1) {
       Float_t  vth = 0; Float_t triple = 0.; Float_t quadruple = 0; Float_t singole = 0.;
       inputFile >> vth >> triple >> quadruple >> singole;
       if (!inputFile.good()) break;
       Float_t eff = quadruple/triple;
       Float_t deff = sqrt(eff*(1.-eff)/triple);
       cout<<"reading from file "<<fileName<<" vth "
           <<vth<<" t/q/s  "<<triple<<" "<<quadruple<<" "<<singole<<endl;
       cout<<"reading from file "<<fileName<<" vth "
           <<vth<<" eff "<<eff<<"+/-"<<deff<<" single rate "<<singole<<endl;
       // qui si riempiono vettori di eff, err-su-eff,
       // rate di singola e poi li si plotta come i vettori in basso
       x[nP]      = vth;
       dx[nP]     = 0.001; // trascuriamo l'rrore sulla tensione di discriminazione
       eff_v[nP]  = eff;
       deff_v[nP] = deff;
       nP++;
   }
   

   gr = new TGraphErrors(nP,x,eff_v,dx,deff_v);
   gr->SetTitle("scintillatore n. xxx - discrimination thr. 18 mV");
   gr->SetMarkerColor(4);
   gr->SetMarkerStyle(21);
   gr->GetXaxis()->SetTitle("HV / kV");
   gr->GetYaxis()->SetTitle("efficiency");
//gr->Draw("ALP");
   gr->Draw("AP");

   c1->Update();
}