Setting the x margin in a plot of a data frame with a DatetimeIndex is ignored.
from [mailing list](https://groups.google.com/forum/?fromgroups=#!searchin/pystatsmodels/pandas$20plot$20with$20DatetimeIndex$20ignore$20x$20axis$20margin/pystatsmodels/9ZQkzoSibSE/jvU35qVeasUJ)

Hello,
I noticed that setting the x margin in a plot of a data frame with a DatetimeIndex is ignored.

Example plot: 

![Example plot](http://i.imgur.com/ZOqmJ.png)

``` python
#!/usr/bin/python

from matplotlib.pyplot import *
from pandas import *
from pylab import show

fig, axes = subplots(2,1)

df1 = DataFrame([0.5,1,1.5,2,2.5,3,3.5])
df1.plot(ax=axes[0],drawstyle='steps',style='g')

# this x axis margin is honored
axes[0].margins(0.05,0.05)


df2 = DataFrame([0.5,1.5,2.5,3.5],index=date_range('1-1-2000',periods=4))
df2.plot(ax=axes[1],drawstyle='steps',style='r')

# this x axis margin is ignored
axes[1].margins(0.05,0.05)

show()
```
