using namespace RooFit;
void first(){
  TFile * f = new TFile("first.root", "Read");
  TTree * t = (TTree*)f -> Get("t1");
  TCanvas * c1 = new TCanvas("c1", "c1", 1);
  t->Draw("var1");
  RooRealVar var1("var1","var1",-2,2);
  
  //DICHIARO IL DATASET
  RooDataSet data("data","data", t, RooArgSet(var1));
  
  //DICHIARO IL RooPlot e  DSEGNO IL RooDataSet
  RooPlot * frame = var1.frame();
  data.plotOn(frame, Name("Hist"));
  frame->Draw();  
  
  TCanvas * c2 = new TCanvas("c2","c2",1);

  
  
  RooRealVar par1("par1","par1",0.7,-0.9, 0.9);
  RooRealVar par2("par2","par2",0.5,-0.9, 0.9);
  
  //DICHIARO LA pdf
  RooChebychev pdf("pdf","pdf",var1, RooArgList(par1,par2));
  pdf.fitTo(data,Minos(kFALSE));
  
  
  //DISEGNO IL DATASET E LE pdf sul RooPlot
  data.plotOn(frame);
  pdf.plotOn(frame, Name("FitCurve"));
  pdf.paramOn(frame);

  frame->Draw();
  
  Double_t  chi2 = frame->chiSquare("FitCurve", "Hist");
  cout<<"chi2 = " << chi2 << endl;
	  
}