Pandas does not respect shared axes in matplotlib
#### Code Sample, a copy-pastable example if possible

x axis truncated by second call

```
ser1 = pd.Series({0:1, 1:2, 2:3})
ser2 = pd.Series({0:.5, 1:.1})

_, axes = plt.subplots(1, 2, sharex=True, sharey=True)

ser1.plot(ax=axes[0])
ser2.plot(ax=axes[1])
```

![image](https://cloud.githubusercontent.com/assets/616139/17912487/c7c800da-6948-11e6-9a40-2a18b737fb98.png)
#### Expected Output

Using only matplotlib: 

```
ser1 = pd.Series({0:1, 1:2, 2:3})
ser2 = pd.Series({0:.5, 1:.1})

_, axes = plt.subplots(1, 2, sharex=True, sharey=True)

axes[0].plot(ser1.index, ser1.values)
axes[1].plot(ser2.index, ser2.values)

```

![image](https://cloud.githubusercontent.com/assets/616139/17912473/aae07ad8-6948-11e6-871e-fbedbec94b1f.png)
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.10.final.0
python-bits: 64
OS: Darwin
OS-release: 14.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 25.1.0
Cython: 0.24
numpy: 1.11.1
scipy: 0.17.1
statsmodels: 0.6.1
xarray: None
IPython: 4.2.1
sphinx: 1.3.5
patsy: 0.4.1
dateutil: 2.2
pytz: 2016.6.1
blosc: None
bottleneck: None
tables: None
numexpr: 2.4.6
matplotlib: 1.5.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.14
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
```
