import numpy as np
import ROOT

npoints = 3
xaxis = np.array([1.0, 2.0, 3.0])
yaxis = np.array([10.0, 20.0, 30.0])

gr1 = ROOT.TGraph(npoints, xaxis, yaxis)
ff = ROOT.TF2("ff", "-1./y")  # Defining the function `f`

c1 = ROOT.TCanvas("c1", "c1", 0, 0, 700, 500)
c1.Divide(2, 1)

c1.cd(1)
gr1.DrawClone("A*")  # Using DrawClone to create a copy of the graph in the canvas.
c1.cd(2)
gr1.Apply(ff)  # Applies the function `f` to all the data TGraph points
gr1.Draw("A*")
# Without DrawClone, the modifications to gr1 via Apply(ff) are reflected in the original graph
# displayed in c1 (the two drawn graphs are not independent).