Double_t fitfunc(Double_t *x,Double_t *par) {
   return par[0]+par[1]*x[0]+par[2]*x[0]*x[0];
}


void prova()
{
   TF1 func("func",fitfunc,-4,4,3);
   func.SetLineColor(kRed);
   gStyle->SetOptFit(1101);
   TH1F *h = new TH1F("h","h",100,-4,4);
   h->FillRandom("pol2");
   TCanvas *canv = new TCanvas("canv","fit");
   canv->cd();
   h->Fit("func","Q");
   h->Draw();
   canv->Update();
   canv->Print("data.root");
}