BUG in plotting timeseries data with twinx (different data representation on each ax)
Simplified the example.
During issue reporting i updated my pandas version from 0.13 to 0.18.1 - with version 0.13 i was able to add the whole DataFrame to my plot, only after i selected some rows between dates it produces the error. After the update the behaviour became worse - also adding the whole DataFrame to the plot now produces the error.
#### A small, complete example of the issue

``` python
import pandas
import dateutil.parser
import matplotlib.pyplot as plt

p_vals = {
    'x_vals' : [
        "2006-12-17 00:00:00+01:00",
        "2006-12-18 00:00:00+01:00",
        "2006-12-19 00:00:00+01:00",
        "2006-12-20 00:00:00+01:00",
        "2006-12-21 00:00:00+01:00",
        "2006-12-22 00:00:00+01:00",
        "2006-12-23 00:00:00+01:00",
        "2006-12-24 00:00:00+01:00",
        "2006-12-25 00:00:00+01:00",
        "2006-12-26 00:00:00+01:00",
    ],
    'y_vals' : [
        10,9,8,7,6,5,4,3,2,1
    ]
}

p_vals2 = {
    'x_vals' : [
        "2006-12-17 00:00:00+01:00",
        "2006-12-18 00:00:00+01:00",
        "2006-12-19 00:00:00+01:00",
        "2006-12-20 00:00:00+01:00",
        "2006-12-21 00:00:00+01:00",
    ],
    'y_vals' : [
        1,2,3,4,5
    ]
}

p_vals['x_vals'] = [ dateutil.parser.parse(x) for x in p_vals['x_vals'] ]
p_vals2['x_vals'] = [ dateutil.parser.parse(x) for x in p_vals2['x_vals'] ]

df = pandas.DataFrame(data = [1,2,3,4,5], index=["2006-12-17","2006-12-18","2006-12-19","2006-12-20","2006-12-21"])
df.index = pandas.to_datetime(df.index, format="%Y-%m-%d")

fig, ax1 = plt.subplots()
ax2 = ax1.twinx()

ax1.plot(p_vals['x_vals'], p_vals['y_vals'], color="r")
#ax2.plot(p_vals2['x_vals'], p_vals2['y_vals'], color="b") # works as intended, see second attached image
df.plot(ax=ax2, color="b") # hides data on ax1, see first image

plt.show()

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

<details>
## INSTALLED VERSIONS

commit: None
python: 3.4.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.19.0-69-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
nose: 1.3.1
pip: 8.1.2
setuptools: 3.3
Cython: None
numpy: 1.11.1
scipy: 0.13.3
statsmodels: None
xarray: None
IPython: 1.2.1
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: None
tables: 3.1.1
numexpr: 2.2.2
matplotlib: 1.5.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.2.1
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: None
pandas_datareader: None
![pandas_how_it_looks](https://cloud.githubusercontent.com/assets/5508360/19013316/a0f93098-87cd-11e6-9450-b1184d3a3b44.png)
![pandas_how_it_should_look](https://cloud.githubusercontent.com/assets/5508360/19013317/a595e484-87cd-11e6-8a6a-a2b842bff8a9.png)

</details>
