{
    TRandom3 rnd;
    const int NFILL = 100000;
    
    TH1D *h1 = new TH1D("h1","Poisson data",20,-0.5,19.5);
    h1->SetBarWidth(0.3);
    TH1D *h2 = (TH1D *)h1->Clone("h2");
    TH1D *h3 = (TH1D *)h1->Clone("h3");

    for(int i=0;i<20;i++) {
        double mu = 5.0;
        h1->SetBinContent(i+1,pow(mu,i)*exp(-mu)/TMath::Factorial(i)*NFILL);
    }
    for(int i=0; i<NFILL; i++) {
        int n1 = rnd.Poisson(5.0);
        h2->Fill(n1);

        int n2 = rnd.Poisson(2.5)+rnd.Poisson(1.5)+rnd.Poisson(1.0);
        h3->Fill(n2);
    }
    
    h1->SetStats(0);
    h1->SetFillColor(kBlack);
    h1->Draw("bar");
    h2->SetBarOffset(0.30);
    h2->SetFillColor(kRed);
    h2->Draw("barsame");
    h3->SetBarOffset(0.60);
    h3->SetFillColor(kBlue);
    h3->Draw("barsame");
    
    TLegend *leg1 = new TLegend(0.45,0.65,0.88,0.88);
    leg1->AddEntry(h1,"h1 Poisson Prob.","F");
    leg1->AddEntry(h2,"h2 Poisson Generation","F");
    leg1->AddEntry(h3,"h3 Sum of Poisson","F");
    leg1->Draw();
}