Make bar plot on secondary axis, the line plot on the primary axis wont show.
Following issue  #3598,

```
df = pd.DataFrame(np.random.uniform(size=10).reshape(5,2),columns=['A','B'])
df['A'] = df['A'] * 100
df.A.plot()
df.B.plot(kind='bar',secondary_y=True)
plt.show()
```

This gives the right plot, with `A` plotted as line in the primary axis, and `B` is plotted as bar in the secondary axis.

But if I index the dataframe by datetime

```
df = pd.DataFrame(np.random.uniform(size=10).reshape(5,2),columns=['A','B'])
df = df.set_index(pd.date_range('20130101',periods=5))
df['A'] = df['A'] * 100
df.A.plot()
df.B.plot(kind='bar',secondary_y=True)
plt.show()
```

This wont plot A as line in the primary axis, only the B as bar in the secondary axis. Not sure why.
