DataFrame.plot overrides previous plots if the frame's index is DatetimeIndex
Environment is Anaconda 2.3, using python 2.7.10. Pandas version 0.16.2.

Here's the code to reproduce this problem in IPython Notebook 3.2, only `df['col3']` is plotted. 

When sharing an axis the last plot() seems to override whatever was plotted before. Only the last plotted item is shown in figure.

```
import pandas as pd
import bumpy as np
import matplotlib.pyplot as plt
%matplotlib inline

drange = pd.date_range(start='2015-07-01', end='2015-07-10', freq='D')
df = pd.DataFrame(data=np.random.random_sample((10, 3)), index=drange, columns=['col1', 'col2', 'col3'])
fig, ax = plt.subplots()
plt.hold(True) # when the index is just integer type, this will make it work.
df[['col1', 'col2']].plot(kind='bar', ax=ax, stacked=True)
df[['col3']].plot(ax=ax, color='r', marker='o', ls='-', alpha=.7) # This overrides the previous plot
# if you use this line below instead of the line above, both bars and lines will show up:
#ax.plot(df[['col3']], color='r', marker='o', ls='-', alpha=.7)
ax.grid(True)
ax.legend(loc='best')
```

![bug1](https://cloud.githubusercontent.com/assets/7330385/9144475/1dc4e8da-3d45-11e5-938f-fc99b080955d.png)
