BUG: Change of behavior between 1.1.5 and 1.2.0 in plot functions
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import pandas as pd
from pandas import Timestamp
import matplotlib.pyplot as plt

a= {'Argentina': {Timestamp('2020-11-17 00:00:00'): 56.0},
 'Brazil': {Timestamp('2020-11-17 00:00:00'): 103.0},
 'Guatemala': {Timestamp('2020-11-17 00:00:00'): 99.0},
 'Ireland': {Timestamp('2020-11-17 00:00:00'): 53.0},
 'Italy': {Timestamp('2020-11-17 00:00:00'): 19.0},
 'Spain': {Timestamp('2020-11-17 00:00:00'): 49.0},
 'Sweden': {Timestamp('2020-11-17 00:00:00'): 42.0},
 'US': {Timestamp('2020-11-17 00:00:00'): 86.0},
 'United Kingdom': {Timestamp('2020-11-17 00:00:00'): 32.0}}

b = {'Val': {'Afghanistan_confirmed': 50886},
 'Country': {'Afghanistan_confirmed': 'Afghanistan'},
 'Population': {'Afghanistan_confirmed': 37.171922},
 'Ratio': {'Afghanistan_confirmed': 1368.9364784527418}}
doubling_df = pd.DataFrame().from_dict(a)
df = pd.DataFrame().from_dict(b)
_ = doubling_df.plot()
_ = df.sort_values(by='Ratio', ascending=False)[:1]['Ratio'].plot.bar(color='blue')


```

#### Stack Trace

<details>

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/matplotlib/axis.py", line 1523, in convert_units
    ret = self.converter.convert(x, self.units, self)
  File "/usr/local/lib/python3.8/site-packages/matplotlib/category.py", line 61, in convert
    unit.update(values)
  File "/usr/local/lib/python3.8/site-packages/matplotlib/category.py", line 211, in update
    cbook._check_isinstance((str, bytes), value=val)
  File "/usr/local/lib/python3.8/site-packages/matplotlib/cbook/__init__.py", line 2246, in _check_isinstance
    raise TypeError(
TypeError: 'value' must be an instance of str or bytes, not a datetime.datetime

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/pablo/bug_report.py", line 22, in <module>
    _ = df.sort_values(by='Ratio', ascending=False)[:1]['Ratio'].plot.bar(color='blue')
  File "/usr/local/lib/python3.8/site-packages/pandas/plotting/_core.py", line 1113, in bar
    return self(kind="bar", x=x, y=y, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/pandas/plotting/_core.py", line 955, in __call__
    return plot_backend.plot(data, kind=kind, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/pandas/plotting/_matplotlib/__init__.py", line 61, in plot
    plot_obj.generate()
  File "/usr/local/lib/python3.8/site-packages/pandas/plotting/_matplotlib/core.py", line 280, in generate
    self._make_plot()
  File "/usr/local/lib/python3.8/site-packages/pandas/plotting/_matplotlib/core.py", line 1433, in _make_plot
    ax.xaxis.update_units(self.ax_index)
  File "/usr/local/lib/python3.8/site-packages/matplotlib/axis.py", line 1466, in update_units
    default = self.converter.default_units(data, self)
  File "/usr/local/lib/python3.8/site-packages/matplotlib/category.py", line 107, in default_units
    axis.set_units(UnitData(data))
  File "/usr/local/lib/python3.8/site-packages/matplotlib/axis.py", line 1541, in set_units
    self.callbacks.process('units')
  File "/usr/local/lib/python3.8/site-packages/matplotlib/cbook/__init__.py", line 229, in process
    self.exception_handler(exc)
  File "/usr/local/lib/python3.8/site-packages/matplotlib/cbook/__init__.py", line 81, in _exception_printer
    raise exc
  File "/usr/local/lib/python3.8/site-packages/matplotlib/cbook/__init__.py", line 224, in process
    func(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/matplotlib/lines.py", line 648, in recache_always
    self.recache(always=True)
  File "/usr/local/lib/python3.8/site-packages/matplotlib/lines.py", line 652, in recache
    xconv = self.convert_xunits(self._xorig)
  File "/usr/local/lib/python3.8/site-packages/matplotlib/artist.py", line 175, in convert_xunits
    return ax.xaxis.convert_units(x)
  File "/usr/local/lib/python3.8/site-packages/matplotlib/axis.py", line 1525, in convert_units
    raise munits.ConversionError('Failed to convert value(s) to axis '
matplotlib.units.ConversionError: Failed to convert value(s) to axis units: array([datetime.datetime(2020, 11, 17, 0, 0)], dtype=object)

</details>

#### Problem description

There was some change in behavior between pandas 1.1.5 and 1.2.0 where the code above crashes with the stack trace detailed. The solution for 1.2.0 to work is to add a ```plt.close('all')``` between the two plot functions, but this change of behavior is not documented in the release guide so it seems to be a bug.

This happens only when running this in a headless python setup (in the real code I use ```get_figure``` to save the images) - when running in a Jupyter notebook this code works as expected. 

#### Expected Output

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 3e89b4c4b1580aa890023fc550774e63d499da25
python           : 3.8.7.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.121-linuxkit
Version          : #1 SMP Tue Dec 1 17:50:32 UTC 2020
machine          : x86_64
processor        :
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.0
numpy            : 1.19.4
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 51.1.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.4
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
