make the align keyword work like matplotlib in bar plots
[SO question](http://stackoverflow.com/questions/18151775/create-yoy-graph-with-matplotlib/18153525#18153525)

if you do it with pandas like this:

```
In [60]: s.plot(kind='bar', align='center')
Out[60]: <matplotlib.axes.AxesSubplot at 0xc1f3810>

In [61]: ax = gca()

In [62]: ax2 = twinx(ax)

In [63]: s.plot(ax=ax2)
Out[63]: <matplotlib.axes.AxesSubplot at 0xd1ffc50>
```

it overwrites the bar plot
