pandas scatter plot: Secondary axis overrides primary
When a second scatter plot is added as ''secondary_y'', the outcome is that the primary is overridden.

``` python
import matplotlib.pyplot as plt
import pandas as pd,numpy as np

# secondary axis does not work on scatter plot
x=pd.Series(np.linspace(-10,10,201))
y,z=np.sin(x),np.cos(x)
df=pd.concat([x,np.sin(x),np.cos(x)],axis=1)
df.columns=['x','sin(x)','cos(x)']
ax=df.plot(kind='scatter',x='x',y='sin(x)',label='sin(x)',color='blue')
df.plot(kind='scatter',x='x',y='cos(x)',label='cos(x)',color='red', \
    ax=ax,secondary_y=True)
plt.show()
```

The above code returns this plot.

![pandas_bug6_1](https://cloud.githubusercontent.com/assets/10504477/7168086/ae5077aa-e388-11e4-89a5-b7aa92b04a26.png)

This could be a workaround, however I think that the first snippet should have worked.

``` python
ax=df.plot(kind='scatter',x='x',y='sin(x)',label='sin(x)',color='blue')
ax2=ax.twinx()            
df.plot(kind='scatter',x='x',y='cos(x)',label='cos(x)',color='red', \
    ax=ax2,secondary_y=True)
plt.show()
```

![pandas_bug6_2](https://cloud.githubusercontent.com/assets/10504477/7168089/b7478524-e388-11e4-9895-ff4791677809.png)
