Horizonal line at zero with SetDefaultSumw2

ROOT
1

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?

2

Yes i get this line also with:

{
   TH1::SetDefaultSumw2(kTRUE);
   TGraph g;
   g.SetPoint(0,-1,-1);
   g.SetPoint(1,1,1);
   g.Draw();
   g.Draw();
}