TST: avoid matplotlib warnings in tests

[jbrockmendel]

In master we get the following warnings in this file:

$ pytest pandas/tests/plotting/test_frame.py --skip-slow
[...]

pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_mpl2_color_cycle_str
  pandas/pandas/plotting/_style.py:60: MatplotlibDeprecationWarning: Support for uppercase single-letter colors is deprecated since Matplotlib 3.1 and will be removed in 3.3; please use lowercase instead.
    [conv.to_rgba(c) for c in colors]

pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_subplots_timeseries_y_axis
  pandas/pandas/plotting/_core.py:384: FutureWarning: Converting timezone-aware DatetimeArray to timezone-naive ndarray with 'datetime64[ns]' dtype. In the future, this will return an ndarray with 'object' dtype where each element is a 'pandas.Timestamp' with the correct 'tz'.
  	To accept the future behavior, pass 'dtype=object'.
  	To keep the old behavior, pass 'dtype="datetime64[ns]"'.
    numeric_data[col] = np.asarray(numeric_data[col])

pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
  pandas/pandas/plotting/_core.py:1014: UserWarning: Attempted to set non-positive left xlim on a log-scaled axis.
  Invalid limit will be ignored.
    ax.set_xlim(left, right)

This PR addresses first and third of these. The middle one is the topic of #26676.

[alimcmaster1]

Interesting I saw the same error in my PR:

   raise_assert_detail(obj, '{0} length are different'.format(obj),
E   AssertionError: Iterable are different
E   
E   Iterable length are different
E   [left]:  133
E   [right]: 128
E   [diff]: ['e', 'msg', 'missing_dependencies', 'dependency', 'hard_dependencies']

https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=12339

Might raise an issue?

[codecov]

Codecov Report

Merging #26678 into master will decrease coverage by 50.1%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master   #26678       +/-   ##
===========================================
- Coverage   91.88%   41.78%   -50.11%     
===========================================
  Files         174      174               
  Lines       50694    50663       -31     
===========================================
- Hits        46581    21168    -25413     
- Misses       4113    29495    +25382

Flag	Coverage Δ
#multiple	?
#single	41.78% <ø> (-0.21%)	⬇️

Impacted Files	Coverage Δ
pandas/io/formats/latex.py	0% <0%> (-100%)	⬇️
pandas/io/sas/sas_constants.py	0% <0%> (-100%)	⬇️
pandas/core/groupby/categorical.py	0% <0%> (-100%)	⬇️
pandas/tseries/plotting.py	0% <0%> (-100%)	⬇️
pandas/tseries/converter.py	0% <0%> (-100%)	⬇️
pandas/io/formats/html.py	0% <0%> (-99.37%)	⬇️
pandas/io/sas/sas7bdat.py	0% <0%> (-91.16%)	⬇️
pandas/io/sas/sas_xport.py	0% <0%> (-90.1%)	⬇️
pandas/core/sparse/scipy_sparse.py	10.14% <0%> (-89.86%)	⬇️
pandas/core/tools/numeric.py	10.14% <0%> (-89.86%)	⬇️
... and 128 more

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[codecov]

Codecov Report

Merging #26678 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #26678      +/-   ##
==========================================
- Coverage   91.88%   91.87%   -0.01%     
==========================================
  Files         174      174              
  Lines       50699    50699              
==========================================
- Hits        46587    46582       -5     
- Misses       4112     4117       +5

Flag	Coverage Δ
#multiple	90.41% <ø> (ø)	⬆️
#single	41.91% <ø> (-0.11%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	97% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.6% <0%> (-0.11%)	⬇️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[jbrockmendel]

@alimcmaster1 is the issue you mentioned affecting all PRs or just these two?

[jorisvandenbossche]

@alimcmaster1 is the issue you mentioned affecting all PRs or just these two?

It was a temporary failure (due to me merging something that should not have been merged) that is fixed by now.

[TomAugspurger]

Thanks!