matplotlib's xaxis "margin" dropped when plotting using pandas
#### Code Sample

```python
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
x = np.arange(10)
y = x
fig, axs = plt.subplots(2, 1, constrained_layout=True)
axs[0].plot(x, y, linestyle='', marker='.')
axs[0].set_title('matplotlib')
pd.Series(y, index=x).plot(ax=axs[1], linestyle='', marker='.')
axs[1].set_title('pandas')
fig.show()
```

#### Problem description

![Figure_1](https://user-images.githubusercontent.com/7373718/73926764-70bc2f80-48d0-11ea-8cf9-2f6a1502eec8.png)


Matplotlib adds some "margin" on both sides of the horizontal axis, but pandas does not.

This is a problem because, when we plot using points, the extreme x values are shown confused with the vertical axis lines, which makes them very difficult to see.

Would it be possible to align pandas' behavior with matplotlib's one?

Thanks a lot.

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

<details>

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.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_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.3
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.1.0
Cython           : 0.29.14
pytest           : None
hypothesis       : None
sphinx           : 2.2.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.2
html5lib         : None
pymysql          : None
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : 2.10.3
IPython          : 7.10.1
pandas_datareader: None
bs4              : 4.8.1
bottleneck       : 1.3.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.2
matplotlib       : 3.1.2
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.1
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.3
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
```
</details>
