pandas.DataFrame.plot(): Labels do not appear in legend
The following code plots two lines. The column names appear in the legend.

``` python
x=np.linspace(-10,10,201)
y,z=np.sin(x),np.cos(x)
x,y,z=pd.Series(x),pd.Series(y),pd.Series(z)
df=pd.concat([x,y,z],axis=1)
df.columns=['x','sin(x)','cos(x)']
df=df.set_index('x')
df.plot()
plt.show()
plt.clf();plt.close()
```

![figure_1](https://cloud.githubusercontent.com/assets/10504477/6354154/a5b15c66-bc1b-11e4-9811-3ad39c033a85.png)

However, the following equivalent code shows **None** as legend, even though the labels are explicitly set.

``` python
ax=df.plot(y='sin(x)',label='sin(x)')
df.plot(y='cos(x)',label='cos(x)',ax=ax)
plt.show()
```

Of course there are alternative ways to make this work, but it appears to me that passing the labels should suffice. In particular, I don't think their values should be replaced with None, or printed as x-label.

![figure_2](https://cloud.githubusercontent.com/assets/10504477/6354161/b137068a-bc1b-11e4-96b0-ba4c093d8c67.png)
