Series.plot misaligned for sparse DatetimeIndex
#### Code Sample, a copy-pastable example if possible

```python
import os.path
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

version = pd.__version__.replace('.', '-')

image_series = 'plot_series_{}.png'.format(version)
image_dataframe = 'plot_dataframe_{}.png'.format(version)

assert not os.path.exists(image_series), 'already ran this version'
assert not os.path.exists(image_dataframe), 'already ran this version'

idx = pd.date_range(start='2014-01-01', end='2016-12-31', freq='D')
np.random.seed(0)
ser = pd.Series(np.random.rand(idx.size) - 0.5, index=idx).cumsum()
rs = ser.resample('A').mean()
af = ser.asfreq('A')
df = pd.DataFrame({'ser': ser, 'rs': rs, 'af': af},
                  columns=['ser', 'rs', 'af'])

fig, ax = plt.subplots(1, 1)
ser.plot(ax=ax, style='o')
rs.plot(ax=ax, style='o')
af.plot(ax=ax, style='o')
fig.savefig(image_series)

fig, ax = plt.subplots(1, 1)
df.plot(ax=ax, marker='o')
fig.savefig(image_dataframe)
```
#### Problem description

When the date in a `DatetimeIndex` is December 31, and it is the only date for the year, `DataFrame.plot` positions the value at the end of the year (yellow = mean, green = last):
![plot_dataframe_0-20-2](https://user-images.githubusercontent.com/13019694/27940667-919cd34e-6291-11e7-971e-d1aaa927f8f3.png)
This is also how `matplotlib.pyplot.plot` works (not shown) when called directly with `x=s.index, y=s`.

`Series.plot`, on the other hand, positions the value at the start of the year:
![plot_series_0-20-2](https://user-images.githubusercontent.com/13019694/27940668-919ecfd2-6291-11e7-98fe-75a0e75c360f.png)

I went down the rabbit hole a bit but couldn't figure out where in the code the index is being converted for `Series` but not for `DataFrame`.

This appears to be consistent across 0.20.2, 0.20.1, and 0.19.2 (I stopped there). Windows and OS X.

#### Expected Output

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Darwin
OS-release: 15.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.2
pytest: 2.9.2
pip: 9.0.1
setuptools: 23.0.0
Cython: 0.24
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 5.1.0
sphinx: 1.4.1
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: 1.2.1
tables: 3.3.0
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.3.2
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.2
lxml: 3.6.0
bs4: 4.4.1
html5lib: None
sqlalchemy: 1.0.13
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
pandas_gbq: None
pandas_datareader: 0.2.1
None
</details>
