This is really strange, ROOT-6.04/02:
import ROOT
# ROOT.TH1.SetDefaultSumw2(True)
canvas_line = ROOT.TCanvas()
g = ROOT.TGraph()
g.SetPoint(0, -1, -1)
g.SetPoint(1, 1, 1)
g.Draw("APL")
canvas_line.SaveAs("canvas_line.png")
Why if I uncomment the commented line I get an horizontal line at 0?