axvspan() sometimes breaks when using matplotlib wrappers
Hi, 

I was trying to highlight some data in a DataFrame using the `axvspan` method from matplotlib and I ran into some ill behavior. Here's a minimal example:

```
import numpy as np
import pandas as pd
import matplotlib.dates as mdates

from pylab import *
ion()

dates = pd.date_range(start="10/01/2013", end="10/04/2013", freq='H')
data = np.random.random_sample(len(dates))

my_df = pd.DataFrame({'r': data}, index=dates)

figure()
my_df.plot()
axvspan(*mdates.datestr2num(["10/02/2013", "10/03/2013"]),
             facecolor='red', alpha=0.5)
```

This produces the following figure:

![vspan_test_bad](https://f.cloud.github.com/assets/4992424/1258469/bcc293be-2bbe-11e3-89c1-37136283602e.png)

However, when I instead invoke the plotter via `my_df.plot(x_compat=True)`, things are kosher:

![vspan_test_good](https://f.cloud.github.com/assets/4992424/1258474/e3cbbf9e-2bbe-11e3-9766-075ad9014710.png)

save for the fact that the advanced x-axis formatting has been lost (easy enough to replace). I'm not sure if this is an unintended feature or a bug, but it's not documented anywhere.
