Plotting secondary axis
This could be either a bug, or an error in the documentation, or just my fault (hopefully the third reason).

Pandas 0.15.2 documentation explains how to create one plot with two axes (http://pandas.pydata.org/pandas-docs/dev/visualization.html?highlight=visualization#plotting-on-a-secondary-y-axis). For example:

``` python
import numpy as np,pandas as pd,matplotlib.pyplot as plt
x=pd.DataFrame(range(100))**.5
y=x+np.random.normal(size=(x.shape[0],1))

# Does not work ()
x.plot()
y.plot(secondary_y=True,style='g:')
plt.show()
```

The problem is, two plots are generated, rather than only one with two axes.

This alternative would work:

``` python
# Works (1 figure, 2 axes)
fig,ax=plt.subplots()
ax2=ax.twinx()
x.plot(ax=ax)
y.plot(ax=ax2,style='g:')
plt.show()
```

Question: Does the example in the documentation actually generate a plot with 2 axes? What I get is two separate plots. This seems to be a bug. If this is not a bug, then I would suggest the example in the documentation is modified to explain the second alternative.
