that might have failed a month ago

It have been failed (on different pages) every time I tried it in last month, three times at Toolforge and about 10 times on my PC while I was debugging my script, none of those runs were successful. That is not a problem that happened once a long time ago. I can't reproduce it on smaller requests, but there are always several pages in incorrect form in this request. Maybe the size of the answer, the generator usage or FlaggedRevs extension are the reasons, I don't now.

Many thanks! matplotlib.use('AGG') makes it works in both python2 and python3. I'm sorry for worrying you, should've googled it manually.

As far as I remember, I've never used virtualenv on tool labs, and bash history proves it. But "import matplotlib.pyplot" line cause no exceptions in python2. On the other hand, I still can't use pyplot, because any operation throws one:

$ python
Python 2.7.6 (default, Oct 26 2016, 20:30:19)
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib.pyplot
>>> import matplotlib.pyplot as plt
>>> plt.figure(figsize=(16, 9), dpi=100)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/pymodules/python2.7/matplotlib/pyplot.py", line 423, in figure
    **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_tkagg.py", line 79, in new_figure_manager
    return new_figure_manager_given_figure(num, figure)
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_tkagg.py", line 87, in new_figure_manager_given_figure
    window = Tk.Tk()
  File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 1767, in __init__
    self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use)
_tkinter.TclError: no display name and no $DISPLAY environment variable
>>> exit()
$ which python
/usr/bin/python

Isorry for misleading you, I just haven't tried to do it in second python before)
Interesting, is it even possible to use GUI library in console application. I don't use any GUI forms, my plots are saved directly to files, but still.