#include <TF1.h>
#include <TMath.h>
#include <TCanvas.h>

//______________________________________________________________________________
Double_t func1(Double_t *arg, Double_t *par = 0)
{
   return TMath::Sin(arg[0]);
}

//______________________________________________________________________________
Double_t func2(Double_t *arg, Double_t *par = 0)
{
   return TMath::Cos(arg[0]);
}

//______________________________________________________________________________
void CheckBuildLegend()
{
   TF1 *Func_1 = new TF1("Sine",  func1,0.,TMath::TwoPi(),0);
   TF1 *Func_2 = new TF1("Cosine",func2,0.,TMath::TwoPi(),0);

   TCanvas *C1 = new TCanvas("C1", "Sine & Cosine", 700,500);
   Func_1->SetLineColor(kBlue); Func_1->SetLineWidth(2);
   Func_2->SetLineColor(kRed);  Func_2->SetLineWidth(2);
   Func_1->Draw("AL"); Func_2->Draw("L");
   C1->BuildLegend(); C1->Modified();
}