TGraph changes the value of point position

[Tomasz_Bold]

Hi,
I have this trvial python script to draw a simple TGraph (distilled from larger chunk of code):

points = [
(0, 2.836967833067155, 12399074.61),
(1, 6.954407211174568, 20542395.5),
(2, 11.970329210518978, 18499490.9),
(3, 19.424693860517632, 34068743.5),
(4, 29.43182028809318, 24889912.3),
(5, 39.414418675302066, 22688035.700000003),
(6, 49.38887514528897, 25505205.04),
(7, 59.363856464667286, 22036243.92),
(8, 69.3345592681046, 18366012.5),
(9, 79.3097616083224, 14811020.4),
(10, 89.28363506909325, 11568560.3),
(11, 103.50409212026426, 37625910.3),
(12, 119.21492822326776, 11485086.8),
(13, 133.24353243118534, 18563865.1),
(14, 153.08841322492458, 8265027.1),
(15, 172.95258012423116, 3801574.7),
(16, 195.74833705900488, 1737539.1),
(17, 227.38171871397918, 372180.6003 ),
(18, 269.56177047343357, 35339.70004 )]

import ROOT

gr = ROOT.TGraph(len(points))
for p, x, y in points:
  print(p,x,y)
  gr.SetPoint(p, x, y)
# OR
# gr.SetPointX(p, x)
# gr.SetPointY(p, y)

gr.Draw("AL*")

The graph is then drawn only when I click the canvas. But it comes out broken (see attached plot).
Point 14 has x and y positions screwed up. Why point 14?

When I add ROOT.gPad.Update() at the end of the script the points come out as expected.
Seems like the “update” event is messing up sth.

---

ROOT Version: 6.24/02 , Python version: 3.9
Platform: MacOs BigSur
Compiler: Apple clang version 12.0.5 (clang-1205.0.22.11)

---

[eguiraud]

Hi @Tomasz_Bold ,
I cannot reproduce the problem. I tried on my own laptop but also on LXPLUS:

$ ssh -XY eguiraud@lxplus.cern.ch
$ source /cvmfs/sft.cern.ch/lcg/views/LCG_100/x86_64-centos7-gcc10-opt/setup.sh
$ python -i repro.py # where repro.py is a copy-paste of the code you provided

displays

What am I doing differently?

Cheers,
Enrico

[Wile_E_Coyote]

Try:

Updating using gPad->Modified() and gPad->Update()

Maybe you are facing a bug with the Cocoa version on Mac which was discovered some time ago. After each “Update”, try to add gSystem->ProcessEvents(); (executed once or two times). BTW. When you post “source code” or “output” here, do remember to enclose them into two lines which contain just three characters ``` (see how your post has been edited above).

[Tomasz_Bold]

Thanks for these hints. Adding ROOT.gSystem.ProcessEvents() did not help. Though, a drawing sequence when the canvas is made first seems to do the job for now. Quite flaky though.

[eguiraud]

Ah so it could be a Cocoa (i.e. MacOS-only) bug. @couet can you reproduce the problem on mac maybe? Or do you have some more insight into the Cocoa bug that Wile E. mentioned?

[couet]

On Mac, if I do:

% python tomasz.py

where tomasz.py is:

points = [
(0, 2.836967833067155, 12399074.61),
(1, 6.954407211174568, 20542395.5),
(2, 11.970329210518978, 18499490.9),
(3, 19.424693860517632, 34068743.5),
(4, 29.43182028809318, 24889912.3),
(5, 39.414418675302066, 22688035.700000003),
(6, 49.38887514528897, 25505205.04),
(7, 59.363856464667286, 22036243.92),
(8, 69.3345592681046, 18366012.5),
(9, 79.3097616083224, 14811020.4),
(10, 89.28363506909325, 11568560.3),
(11, 103.50409212026426, 37625910.3),
(12, 119.21492822326776, 11485086.8),
(13, 133.24353243118534, 18563865.1),
(14, 153.08841322492458, 8265027.1),
(15, 172.95258012423116, 3801574.7),
(16, 195.74833705900488, 1737539.1),
(17, 227.38171871397918, 372180.6003 ),
(18, 269.56177047343357, 35339.70004 )]

import ROOT

c1 = ROOT.TCanvas()

gr = ROOT.TGraph(len(points))
for p, x, y in points:
  print(p,x,y)
  gr.SetPoint(p, x, y)

gr.Draw("AL*")
c1.SaveAs("plot.png")

I get the correct plot: