Plotting Int64 columns with nulled integers (NAType) fails
#### Code Sample, a copy-pastable example if possible

```
import pandas as pd
import numpy as np

df = pd.DataFrame({'A': [1, 2, 3, 4, 5], 'B': [7, 5, np.nan, 3, 2]})
df.plot(x='A', y='B')
df = df.astype('Int64')
df.plot(x='A', y='B')
```
#### Problem description

The first plotting command works, the second throws the error message

> TypeError: float() argument must be a string or a number, not 'NAType'

#### Expected Output

NAType should be treated the same way as numpy nan in plotting. Maybe transformed on the fly?

(I'm unsure if this is a pandas, a numpy, or a matplotlib issue, I'm starting here)

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 79 Stepping 1, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : en
LOCALE           : None.None

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.2.0.post20200209
Cython           : 0.29.15
pytest           : None
hypothesis       : None
sphinx           : 2.4.1
blosc            : None
feather          : None
xlsxwriter       : 1.2.7
lxml.etree       : 4.5.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : None
fastparquet      : 0.3.3
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.1.3
numexpr          : None
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : 1.2.7
numba            : 0.48.0

</details>
