Stacked histogram

Hello folks,

I am fairly new to Root and C++,

I am currently attempting to use Pythia8 with Delphes and Root and one of their example files is

Simple macro showing how to access branches from the delphes output root file,
loop over events, store histograms in a root file and print them as image files.

root -l examples/Example2.C'("delphes_output.root")'

#include "TH1.h"
#include "TSystem.h"

#ifdef __CLING__
#include "classes/DelphesClasses.h"
#include "external/ExRootAnalysis/ExRootTreeReader.h"
#include "external/ExRootAnalysis/ExRootResult.h"


struct MyPlots
  TH1 *fJetPT[2];
  TH1 *fMissingET;
  TH1 *fElectronPT;


class ExRootResult;
class ExRootTreeReader;


void BookHistograms(ExRootResult *result, MyPlots *plots)
  THStack *stack;
  TLegend *legend;
  TPaveText *comment;

  // book 2 histograms for PT of 1st and 2nd leading jets

  plots->fJetPT[0] = result->AddHist1D(
    "jet_pt_0", "leading jet P_{T}",
    "jet P_{T}, GeV/c", "number of jets",
    50, 0.0, 100.0);

  plots->fJetPT[1] = result->AddHist1D(
    "jet_pt_1", "2nd leading jet P_{T}",
    "jet P_{T}, GeV/c", "number of jets",
    50, 0.0, 100.0);


  // book 1 stack of 2 histograms

  stack = result->AddHistStack("jet_pt_all", "1st and 2nd jets P_{T}");

  // book legend for stack of 2 histograms

  legend = result->AddLegend(0.72, 0.86, 0.98, 0.98);
  legend->AddEntry(plots->fJetPT[0], "leading jet", "l");
  legend->AddEntry(plots->fJetPT[1], "second jet", "l");

  // attach legend to stack (legend will be printed over stack in .eps file)

  result->Attach(stack, legend);

  // book more histograms

  plots->fElectronPT = result->AddHist1D(
    "electron_pt", "electron P_{T}",
    "electron P_{T}, GeV/c", "number of electrons",
    50, 0.0, 100.0);

  plots->fMissingET = result->AddHist1D(
    "missing_et", "Missing E_{T}",
    "Missing E_{T}, GeV", "number of events",
    60, 0.0, 30.0);

  // book general comment

  comment = result->AddComment(0.64, 0.86, 0.98, 0.98);
  comment->AddText("demonstration plot");
  comment->AddText("produced by Example2.C");

  // attach comment to single histograms

  result->Attach(plots->fJetPT[0], comment);
  result->Attach(plots->fJetPT[1], comment);
  result->Attach(plots->fElectronPT, comment);

  // show histogram statisics for MissingET


void AnalyseEvents(ExRootTreeReader *treeReader, MyPlots *plots)
  TClonesArray *branchJet = treeReader->UseBranch("Jet");
  TClonesArray *branchElectron = treeReader->UseBranch("Electron");
  TClonesArray *branchMissingET = treeReader->UseBranch("MissingET");

  Long64_t allEntries = treeReader->GetEntries();

  cout << "** Chain contains " << allEntries << " events" << endl;

  Jet *jet[2];
  MissingET *met;
  Electron *electron;

  Long64_t entry;

  Int_t i;

  // Loop over all events
  for(entry = 0; entry < allEntries; ++entry)
    // Load selected branches with data from specified event

    // Analyse two leading jets
    if(branchJet->GetEntriesFast() >= 2)
      jet[0] = (Jet*) branchJet->At(0);
      jet[1] = (Jet*) branchJet->At(1);


    // Analyse missing ET
    if(branchMissingET->GetEntriesFast() > 0)
      met = (MissingET*) branchMissingET->At(0);

    // Loop over all electrons in event
    for(i = 0; i < branchElectron->GetEntriesFast(); ++i)
      electron = (Electron*) branchElectron->At(i);


void PrintHistograms(ExRootResult *result, MyPlots *plots)


void Example2(const char *inputFile)

  TChain *chain = new TChain("Delphes");

  ExRootTreeReader *treeReader = new ExRootTreeReader(chain);
  ExRootResult *result = new ExRootResult();

  MyPlots *plots = new MyPlots;

  BookHistograms(result, plots);

  AnalyseEvents(treeReader, plots);

  PrintHistograms(result, plots);


  cout << "** Exiting..." << endl;

  delete plots;
  delete result;
  delete treeReader;
  delete chain;


Is there a simple way to modify the code so that the stack output is similar to the presented here (such as the ocean coloured one) rather than covering one another?

There’s not enough ROOT at the right spots to be able to answer this here: I don’t see a ROOT THStack, no ...->Draw() - it’s probably all in Delphes? Could you contact the Delphes people and ask them? They could even reply here…

Thanks, Axel.