plotBeforeAndAfterMLFit.py

import sys
import ROOT

ROOT.gROOT.SetBatch(True)
ROOT.gErrorIgnoreLevel = ROOT.kWarning
ROOT.RooMsgService.instance().setGlobalKillBelow(ROOT.RooFit.ERROR)

filename = sys.argv[1]
f = ROOT.TFile(filename)
w = f.Get('w')

n_bins = 50
binning = ROOT.RooFit.Binning(n_bins,4.7,5.7)

can = ROOT.TCanvas()
plot = w.var('Bmass').frame()
w.data('data_obs').plotOn( plot, binning )

# Load the S+B model
sb_model = w.pdf('model_s').getPdf('psi2s_region')
b_model =  w.pdf('model_b').getPdf('psi2s_region')

# Prefit
sb_model.plotOn(plot, ROOT.RooFit.LineColor(2), ROOT.RooFit.Name('prefit'))

# Postfit
w.loadSnapshot('MultiDimFit')
sb_model.plotOn(plot, ROOT.RooFit.LineColor(4), ROOT.RooFit.Name('postfit'))
b_model.plotOn(plot, ROOT.RooFit.LineColor(3), ROOT.RooFit.Name('postfit'))
r_bestfit = w.var('r').getVal()

plot.Draw()

leg = ROOT.TLegend(0.65,0.65,0.88,0.88)
leg.AddEntry('prefit', 'Prefit S+B model (r=1.00)', 'L')
leg.AddEntry('postfit', 'Postfit S+B model (r=%.2f)'%r_bestfit, 'L')
leg.Draw('Same')

can.Update()
can.SaveAs(filename.rstrip('.root')+'.beforeAndAfter.png')

f.Close()