Checking plot labels works locally but not during CI

[MarcoGorelli]

Code Sample, a copy-pastable example if possible

xref #31207

If I modified the test as follows (with import re at the top of the file)

        with tm.assert_produces_warning(None):
            fig, ax = self.plt.subplots()
            ts.plot(ax=ax)
            fig.canvas.draw()
            labels = [i.get_text() for i in ax.get_xticklabels()]
            # Extract H:M component, check first point is in correct timezone.
            # NOTE: this test could be updated once GH 31548 is fixed,
            # so that the last point is checked as well.
            assert re.findall(r"[^:]?(\d{2}:\d{2})", labels[0])[0] == "00:00"
            _check_plot_works(ts.plot)

then the test worked locally, but not during CI. Looking at the job details for linux py36_local, I could see that labels was

['01 00:01', ...1 00:51', ...]

I don't understand where that extra minute is coming from (nor can I understand why it only appears during CI).

Output of pd.show_versions()

[paste the output of pd.show_versions() here below this line]
INSTALLED VERSIONS

commit : 6e2d3ae
python : 3.7.6.final.0
python-bits : 64
OS : Linux
OS-release : 4.15.0-74-generic
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8

pandas : 0.26.0.dev0+2036.g6e2d3aeb6
numpy : 1.17.3
pytz : 2019.3
dateutil : 2.8.1
pip : 19.3.1
setuptools : 45.0.0.post20200113
Cython : 0.29.14
pytest : 5.3.3
hypothesis : 5.1.5
sphinx : 2.3.1
blosc : None
feather : None
xlsxwriter : 1.2.7
lxml.etree : 4.4.2
html5lib : 1.0.1
pymysql : None
psycopg2 : None
jinja2 : 2.10.3
IPython : 7.11.1
pandas_datareader: None
bs4 : 4.8.2
bottleneck : 1.3.1
fastparquet : 0.3.2
gcsfs : None
lxml.etree : 4.4.2
matplotlib : 3.1.2
numexpr : 2.7.1
odfpy : None
openpyxl : 3.0.1
pandas_gbq : None
pyarrow : 0.15.1
pytables : None
pytest : 5.3.3
pyxlsb : None
s3fs : 0.4.0
scipy : 1.4.1
sqlalchemy : 1.3.12
tables : 3.6.1
tabulate : 0.8.6
xarray : 0.14.1
xlrd : 1.2.0
xlwt : 1.3.0
xlsxwriter : 1.2.7
numba : 0.47.0

[jorisvandenbossche]

@MarcoGorelli I would approach it a bit differently, and instead of testing the labels on the plot (which is very much dependent on matplotlib, they could eg change it a bit accross versions), I would maybe rather test the data values as they are plotted on the axis. Eg you can get those data with list(ax.get_lines())[0].get_xdata() (with ax the return value of df.plot()). Then could eg check that those values are the same whether you plot timezone aware or timezone naive version