Plotting: series.plot() has different x-limits than plt.plot()
#### Code Sample, a copy-pastable example if possible

```python
x = [1,2,3]
y = [4,3,5]
plt.plot(x,y,marker='o');  # x-limits are extended
pd.Series(y, index=x).plot(marker='o');  # x-limits are exactly on the first/last points
```
#### Problem description

When the data is plotted using `Series.plot()`, the x-limits are set on the exactly first and last points of the set. This is different from the behavior of matplotlib's plot, where it gives a bit more space.

pandas: 0.25.1
matplotlib: 3.0.1

![image](https://user-images.githubusercontent.com/1324881/66713125-ebec5980-eda6-11e9-89ba-9f42b1cd5e1c.png)


<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.1.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.1
numpy            : 1.16.4
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 19.2.3
setuptools       : 41.4.0
Cython           : None
pytest           : 5.2.0
hypothesis       : None
sphinx           : 2.2.0
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.1
html5lib         : None
pymysql          : 0.9.3
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.1.1
pandas_datareader: None
bs4              : 4.6.3
bottleneck       : None
fastparquet      : 0.1.6
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.0.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.15.0
pytables         : None
s3fs             : 0.3.4
scipy            : 1.3.1
sqlalchemy       : 1.3.8
tables           : 3.4.4
xarray           : None
xlrd             : 1.1.0
xlwt             : None
xlsxwriter       : None

</details>
