#include "TMultiGraph.h"
#include "TCanvas.h"
#include "TGraph.h"
#include "TLegend.h"

void ExampleBuildLegend(){

  auto c3 = new TCanvas("c3","c3",600, 400);
  auto mg = new TMultiGraph("mg","mg");

  const Int_t size = 10;
  double px[size];
  double py1[size];
  double py2[size];
  double py3[size];
  for ( int i = 0; i <  size ; ++i ) {
    px[i] = i;
    py1[i] = i;
    py2[i] = 2*i;
    py3[i] = 3*i;
  }
  auto gr1 = new TGraphErrors( size, px, py1 );
  gr1->SetName("gr1");
  gr1->SetTitle("graph 1");
  gr1->SetMarkerStyle(21);
  gr1->SetDrawOption("AP");
  gr1->SetLineColor(2);
  gr1->SetLineWidth(4);
  gr1->SetFillStyle(0);

  auto gr2 = new TGraph( size, px, py2 );
  gr2->SetName("gr2");
  gr2->SetTitle("graph 2");
  gr2->SetMarkerStyle(22);
  gr2->SetMarkerColor(2);
  gr2->SetDrawOption("P");
  gr2->SetLineColor(3);
  gr2->SetLineWidth(4);
  gr2->SetFillStyle(0);

  auto gr3 = new TGraph( size, px, py3 );

  gr3->SetName("gr3");
  gr3->SetTitle("graph 3");
  gr3->SetMarkerStyle(23);
  gr3->SetLineColor(4);
  gr3->SetLineWidth(4);
  gr3->SetFillStyle(0);
  mg->Add( gr1 );
  mg->Add( gr2 );
  mg->Add(gr3);
//  gr3->Draw("AP");
  mg->Draw("AP");
  c3->BuildLegend();
}