Dates being wrongly plotted at start of month
#### Code Sample, a copy-pastable example if possible

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

s = pd.Series({
    '2018-04-30':     0,
    '2018-05-31':     1,
    '2018-06-30':     0,
    '2018-07-31':    1,
    '2018-08-31':    0,
    '2018-09-30':    1,
})
s.index = pd.to_datetime(s.index)

# Works as expected
plt.plot(s)

# Mistakenly plots dates at start of month
s.plot()

# It's not just confusing axis-labelling
s2 = pd.Series([0.2, 0.7], index=pd.date_range('2018-05-01', '2018-05-02'))
s.plot()
s2.plot()
```
#### Problem description
I have a timeseries in Pandas where the dates are at the end of the month (see code excerpt above).
When I plot this with matplotlib I get the result I'd expect, with points at the end-of-month and the line starting just before May 2018.

But instead Pandas' native plot function mistakenly plots the points at the start of the month.

I thought perhaps this was just Pandas labelling '30 April' as 'April', but that doesn't seem to be the case, since `s2` above is plotted correctly.

Pictures of the plots I get are at https://stackoverflow.com/questions/49816776/pandas-dates-being-wrongly-plotted-at-start-of-month/49820552

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.candidate.1
python-bits: 64
OS: Linux
OS-release: 4.15.0-2-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 39.0.1
Cython: None
numpy: 1.14.2
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: None
dateutil: 2.7.0
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.0
openpyxl: 2.5.1
xlrd: 1.1.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0b10
sqlalchemy: None
pymysql: None
psycopg2: 2.7.3.2 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
