secondary_y sequence doesn't work
#### Code Sample, a copy-pastable example if possible

```
import pandas
df = pandas.DataFrame({'a': [1, 2, 3], 'b': [4, 5, 6], 'c': [7, 8, 9], 'd': [10, 11, 12]})
df.plot(y=['a', 'b'], secondary_y=['c', 'd'])
```

I only get the primary axis results:
![figure_1](https://cloud.githubusercontent.com/assets/2196196/14935868/cd987ce2-0ee9-11e6-9a89-f1459e38630b.png)
Others seem to be using this - am I missing something?
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 15.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8

pandas: 0.18.0
nose: 1.3.7
pip: 8.1.1
setuptools: 20.7.0
Cython: 0.24
numpy: 1.10.4
scipy: 0.17.0
statsmodels: 0.6.1
xarray: None
IPython: 4.2.0
sphinx: 1.4.1
patsy: 0.4.1
dateutil: 2.5.2
pytz: 2016.3
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.5
lxml: 3.6.0
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.12
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext)
jinja2: 2.8
boto: 2.39.0
```
