Just plot some histograms

plot_stack.py

import ROOT

if __name__ == "__main__":

    common_path = "/users2/mratti/MonoPhotonSL6/Run/looseSR/data-output/"

    samples = ({"filenames": common_path + "user.mgenest.slim.mc12_8TeV.126744.Sherpa_CT10_munugammaPt80.vCOMMON1.0.root",
                "treename": "physics",
                "color": ROOT.kOrange - 9,
                "fill": True,
                "name": "Wmunugamma",
                "title": "W#rightarrow#mu#nu#gamma"},
               {"filenames": common_path + "user.mgenest.slim.mc12_8TeV.126022.Sherpa_CT10_nunugammaPt70.vCOMMON1.0.root",
                "treename": "physics",
                "color": ROOT.kYellow - 9,
                "fill": True,
                "name": "Znunugamma",
                "title": "Z#rightarrow#nu#nu#gamma"},
               {"filenames": common_path + "user.mgenest.slim.mc12_8TeV.158920.MadgraphPythia8_AU2MSTW2008LO_WimpPair_D5_DM10.vCOMMON1.0.root",
                "treename": "physics",
                "fill": False,
                "color": ROOT.kRed,
                "name": "signal",
                "title": "signal"},
               )

    sums_names = [("Znunugamma", "Wmunugamma")]

    luminosity = 20276.9

    region_title = "signal region"
    region_name = "signal_region"
    region_cut = "(metMOD > 150E3 && phPT > 125E3 && abs(phETA) < 1.37)"

    quantity = "metMOD / 1E3"
    quantity_title = "MET [GeV]"
    quantity_binning = 5, 150, 500

    stack = ROOT.THStack("stack" + region_name, region_title + " cut: " + region_cut.replace("&&", " "))
    stack.mem = []
    legend = ROOT.TLegend(0.67, 0.5, 0.9, 0.65)
    legend.SetBorderSize(0)

    for sample in samples:
        histo = ROOT.TH1F("histo_{}_{}".format(region_name, sample["name"]),
                          sample["title"], *quantity_binning)
        chain = ROOT.TChain(sample["treename"])
        chain.Add(sample["filenames"])
        weights = "(weight_pileup * weight_vertex * weight_mc * Xsection * FilterEff *1000. * %f *kfactor / neventsGenerated_MarieHelene)" % luminosity
        # chain.Project(histo.GetName(), quantity, weights + " && " + region_cut)
        selection = weights + "*" + region_cut
        chain.Project(histo.GetName(), quantity, selection)
        histo.GetXaxis().SetTitle(quantity_title)
        stack.Add(histo)
        if sample["fill"]:
            histo.SetFillColor(sample["color"])
            histo.SetLineColor(ROOT.kBlack)
        else:
            histo.SetFillColor(0)
            histo.SetLineColor(sample["color"])
        stack.mem.append(histo)
        legend.AddEntry(histo, "", "F" if sample["fill"] else "L")

        sample["histo"] = histo

    canvas = ROOT.TCanvas("canvas_stack_%s" % region_name,
                          "canvas_stack_%s" % region_name)
    stack.Draw()
    stack.GetXaxis().SetTitle(quantity_title)
    canvas.SetLogy()
    legend.Draw()
    atlas_label = ROOT.TLatex(0.6, 0.8, "#bf{#it{ATLAS}} Simulation")
    atlas_label.SetNDC()
    atlas_label.Draw()
    lumi_label = ROOT.TLatex(0.48, 0.73, "#scale[0.6]{#int} Ldt = %.1f fb^{-1}, #sqrt{s}= 8 TeV" % (luminosity / 1000.))
    lumi_label.SetNDC()
    lumi_label.Draw()

    canvas.Update()
    canvas.SaveAs(canvas.GetName() + ".png")

    for sum_names in sums_names:
        # loop on [("Znunugamma",  "Wmunugamma")]
        histos = []
        titles = []
        for name in sum_names:
            # loop on ("Znunugamma",  "Wmunugamma")
            for sample in samples:
                if sample['name'] == name:
                    histos.append(sample['histo'])
                    titles.append(sample['title'])
                    break
        histo_sum = histos[0]
        for histo in histos[1:]:
            histo_sum.Add(histo)

        histo_sum.SetName("histo_sum_" + "_".join(sum_names))
        histo_sum.SetTitle("sum " + ", ".join(sum_names) + ", " + region_title + " cut: " + region_cut.replace("&&", " "))
        legend = ROOT.TLegend(0.67, 0.5, 0.9, 0.65)
        legend.SetBorderSize(0)
        legend.AddEntry(histo_sum, " + ".join(titles))

        canvas_sum = ROOT.TCanvas("canvas_%s_sum_%s" % (region_name, "_".join(sum_names)))
        canvas_sum.SetLogy()
        histo_sum.Draw()
        legend.Draw()
        canvas_sum.SaveAs(canvas_sum.GetName() + ".png")