BUG: df.hist by column alongside with specified columns
#### Histogram through pandas.DataFrame.hist

```python
import pandas as pd
import numpy as np

df = pd.DataFrame({'length': np.random.normal(0, 1, 1000),
                   'width': np.random.normal(0, 1, 1000),
                   'a': np.random.randint(0, 2, 1000)})

df.hist(column=['length', 'width'], by='a', figsize=(4, 2))
```

#### Problem description
by-column `a` plotted along specified columns `length` and `width` ([link](https://i.stack.imgur.com/khZfd.png)). The full problem is described on this stackoverflow [post](https://stackoverflow.com/questions/67289674/remove-by-column-in-pandas-dataframe-hist).

#### Expected Output
Problem vanishes when by-column is non-numeric ([link](https://i.stack.imgur.com/047eQ.png)). 

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

<details>

INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.7.10.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-142-generic
Version          : #146~16.04.1-Ubuntu SMP Tue Apr 13 09:27:15 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.4
numpy            : 1.19.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.1
setuptools       : 49.6.0.post20210108
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.6.3
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.3
IPython          : 7.14.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 2021.04.0
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.4
numexpr          : None
odfpy            : None
openpyxl         : 3.0.5
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.6.2
sqlalchemy       : 1.4.11
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
numba            : None

</details>
