X axis weirdness with DataFrame.plot(kind='bar')
I encountered this issue when plotting some vertical axes spans on the same axes as a bar plot. Some example code is below to reproduce the issue. When using either scatter or line plots, the spans show up where they are supposed to, but the bar plot is off.

Digging a little deeper, I found that the plot call is setting the xticks to a zero-indexed array with a step size of one while setting the tick labels to the correct values.
If you specify the x ticks in the call to the bar plot, the spans are plotted at the correct position, but the bars are plotted from the same  0-indexed, singly incrementing array.

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

spans = pd.DataFrame(np.array([[10,15]]), columns=['left','right'])
datdf = pd.DataFrame(zip(np.random.rand(15),np.random.rand(15)/10), columns=['foo','bar'], index=np.arange(5,20))

fig, axes = plt.subplots(nrows=3, ncols=1)

for a, ax in enumerate(axes):
    for i, row in spans.iterrows():
        ax.axvspan(row['left'],row['right'],color=(0,0,0,.5),zorder=0)


datdf.plot(ax=axes[0], kind='line') # all is good
datdf.plot(ax=axes[1], kind='bar', stacked=True) # span is wrong
datdf.plot(ax=axes[2], kind='bar', stacked=True, xticks=datdf.index.values) #bars are wrong

ticks = []
ticklabs = []

for a, ax in enumerate(axes):
    print a, 
    print zip(ax.xaxis.get_ticklocs(), [lab.get_text() for lab in ax.xaxis.get_ticklabels()])
```

![image](https://cloud.githubusercontent.com/assets/5347796/10809560/0111e020-7db5-11e5-9ab6-cd0adbeb1559.png)

axes 0
[(5.0, u''), (10.0, u''), (15.0, u''), (20.0, u''), (25.0, u''), (30.0, u''), (35.0, u'')]
axes 1
[(0, u'5'), (1, u'7'), (2, u'9'), (3, u'11'), (4, u'13'), (5, u'15'), (6, u'17'), (7, u'19'), (8, u'21'), (9, u'23'), (10, u'25'), (11, u'27'), (12, u'29'), (13, u'31'), (14, u'33')]
axes 2
[(5, u'5'), (7, u'7'), (9, u'9'), (11, u'11'), (13, u'13'), (15, u'15'), (17, u'17'), (19, u'19'), (21, u'21'), (23, u'23'), (25, u'25'), (27, u'27'), (29, u'29'), (31, u'31'), (33, u'33')]
