x tick labels not shown when using secondary_y=True on figure with multiple subplots
This worked on 0.15.1, but not anymore on 0.15.2:

``` python
import pandas as pd
d = {'A' : [1., 2., 3., 4.], 'B' : [4., 3., 2., 1.], 'C': [5, 1, 3, 4]}
df = pd.DataFrame(d, index=pd.date_range('2014 10 11', '2014 10 14'))

axes = df[['A','B']].plot(subplots=True)
df['C'].plot(ax=axes[0], secondary_y=True)   # x tick labels dissappear when this line is executed
```

No x tick labels are shown when using secondary_y on a subplots within a figure containing multiple subplots.
secondary_y works well when only using one subplot (subplots=False) or when layouting the subplots in columns instead of rows.

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.9.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 69 Stepping 1, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.15.2
nose: 1.3.4
Cython: 0.21.1
numpy: 1.9.1
scipy: 0.14.0
statsmodels: 0.6.1
IPython: 2.3.1
sphinx: 1.2.3
patsy: 0.3.0
dateutil: 2.3
pytz: 2014.10
bottleneck: None
tables: 3.1.1
numexpr: 2.4
matplotlib: 1.4.2
openpyxl: None
xlrd: 0.9.3
xlwt: None
xlsxwriter: 0.6.4
lxml: 3.4.1
bs4: 4.3.2
html5lib: None
httplib2: None
apiclient: None
rpy2: 2.5.2
sqlalchemy: 0.9.8
pymysql: None
psycopg2: None
```
