#include <iostream>
#include <string>
#include <stdio.h>

%jsroot on

TFile *file = TFile::Open("http://opendata.atlas.cern/release/samples/MC/mc_147770.Zee.root");

TTree *tree = (TTree*) file->Get("mini");

UInt_t lepton_n = -1;
tree->SetBranchAddress("lep_n", &lepton_n);

TCanvas *canvas = new TCanvas("Canvas","a first way to plot a variable",800,600);

TH1F *hist = new TH1F("variable","Example plot: Number of leptons",4,0,4);

int nentries, nbytes, i;
nentries = (Int_t)tree->GetEntries();

for (i = 0; i < nentries; i++)
{
    nbytes = tree->GetEntry(i);
    hist->Fill(lepton_n);
}

std::cout << "Done!" << std::endl;

hist->Draw();
canvas->Draw();