Error bars drawn in wrong place if SetLogx

[mwilkins]

THistPainter draws error bars in the wrong place on a pad with SetLogx. Reproducer follows.

---

import ROOT
can = ROOT.TCanvas()
can.SetLogx()

h = ROOT.TH1D('h', 'h', 5, 1, 6)
for i in xrange(1, 6):
    h.SetBinContent(i, i)
h.Draw()
can.Update()  # notice that it looks fine
h.Sumw2()
h.Draw()
can.Update()  # notice that the histogram disappears

h2 = ROOT.TH1D('h2', 'h2', 5, 0, 5)  # now make a histogram with a bin at 0
for i in xrange(1, 6):
    h2.SetBinContent(i, i)
h2.Draw()
can.Update()  # notice that it looks fine
h2.Sumw2()
h2.Draw()
can.Update()  # notice that one bin disappears and the lower limits of each bin extend too low
h2.Draw('h')
can.Update()  # notice that the histogram now appears in the right place with only the error bars in the wrong

---

ROOT Version: master
Platform: macOS
Compiler: Not Provided

---

[mwilkins]

Okay, I was able to track down the source of the problem. THistPainter::PaintErrors failed to adjust for the log scale when calculating the location of the error bars. I have created a JIRA task and submitted a pull request fixing the issue.

[couet]

The following (simpler ) macro is enough to show the problem.

void graphlogx() {
   auto can = new TCanvas();
   can->SetLogx();
   auto h2 = new TH1D("h2", "h2", 5, 0, 5);
   for (int i=1; i<6; i++) h2->SetBinContent(i, i);
   h2->Sumw2();
   h2->Draw();
   h2->Draw("hist same");
}

[mwilkins]

Yes, thank you, that’s much less meandering than my script above.

[couet]

I applied this patch. Thanks to @mwilkins to have providing it. It is correct. I wonder why this has not been seen earlier …