import ROOT

ROOT.gROOT.SetStyle("Plain")

x = ROOT.RooRealVar("x", "x", -10, 10)

mu = ROOT.RooRealVar("mu", "average", 0, -1, 1)
sigma = ROOT.RooRealVar("sigma", "r.m.s", 1, 0, 5)
gauss = ROOT.RooGaussian("gauss","gaussian PDF", x, mu, sigma)

lamb = ROOT.RooRealVar("lambda", "exponential slope", -0.1)
expo = ROOT.RooExponential("expo", "exponential PDF", x, lamb)

f = ROOT.RooRealVar("f", "gaussian fraction", 0.5, 0, 1)

sum = ROOT.RooAddPdf("sum", "g + e", ROOT.RooArgList(gauss, expo), ROOT.RooArgList(f))

xFrame = x.frame()
sum.plotOn(xFrame, ROOT.RooFit.LineColor(ROOT.kRed))
sum.plotOn(xFrame, ROOT.RooFit.Components("expo"), ROOT.RooFit.LineColor(ROOT.kBlue))
c = ROOT.TCanvas()
xFrame.Draw()
c.SaveAs("02-gaussianPlusExpo.pdf")