matplotlib autoformat issue when directly calling plt.plot
#### Failed to format x-axis as Timestamp

```
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
ts = pd.Series(np.random.randn(1000), index=pd.date_range('1/1/2000', periods=1000))
plt.plot(ts.index,ts)
plt.show()
```

x-axis is show as raw number of epoch time in nanoseconds (~1.5e18). However, if we call `ts.plot()` once before calling `plt.plot`, x-axis is correctly formatted and the correct behavior will persist for the entire python session, even if we're creating new Series or DataFrame.
I recently upgraded from pandas 0.17.1 to 0.21.0 and the older version did not have this issue.

#### Installed Versions
```
pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.4.3.final.0
python-bits: 64
OS: Linux
OS-release: 2.6.32-642.11.1.el6.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.0
pytest: None
pip: 9.0.1
setuptools: 20.1.1
Cython: None
numpy: 1.13.3
scipy: 0.17.0
pyarrow: None
xarray: None
IPython: 4.1.1
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.0.0
tables: None
numexpr: 2.6.4
feather: None
matplotlib: 2.1.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
```
