BUG/CLN: Repeated time-series plot may raise TypeError
This repeated time-series plotting works:

```
import pandas.util.testing as tm
s1 = tm.makeTimeSeries()
s2 = s1[[0, 5, 10, 11, 12, 13, 14, 15]]
ax = s1.plot()
ax2 = s2.plot(style='g')
```

![line_incorrectsecondary](https://cloud.githubusercontent.com/assets/1696302/7101339/f74b566a-e08f-11e4-8b7f-5ed58327cc5c.png)

But if converted to `DateFrame`, it doesn't:

```
s1 = s1.to_frame()
s2 = s1.iloc[[0, 5, 10, 11, 12, 13, 14, 15]]
print(s2.index.freq)
ax = s1.plot()
ax2 = s2.plot(style='g', ax=ax)
# TypeError: expected string or buffer
```

Fixed the problem, and cleaned up the code to merge `Series` and `DataFrame` flows.
#### After the Fix:

```
import pandas.util.testing as tm
fig, axes = plt.subplots(2, 1)
s1 = tm.makeTimeSeries()
s2 = s1[[0, 5, 10, 11, 12, 13, 14, 15]]
ax = s1.plot(ax=axes[0])
ax2 = s2.plot(style='g', ax=axes[0])

s1 = s1.to_frame(name='x')
s2 = s1.iloc[[0, 5, 10, 11, 12, 13, 14, 15]]
ax = s1.plot(ax=axes[1])
ax2 = s2.plot(style='g', ax=axes[1])
```

![line_incorrectsecondary](https://cloud.githubusercontent.com/assets/1696302/7103495/55f95316-e0e4-11e4-8086-6aedfae82006.png)
