GroupBy plots do not use appropriate xtick values
I have a straightforward little DataFrame that I want to group and plot.

```
    yearID  Krate   teamID
37197    2006    0.115385    COL
37890    2007    0.235294    COL
38621    2008    0.242595    COL
39320    2009    0.264352    COL
40028    2010    0.279373    COL
40738    2011    0.276347    COL
40739    2011    0.275556    CLE
41463    2012    0.225908    CLE
42183    2013    0.307448    CLE
```

However, when I do so, the resulting plot does not put the proper values on the x-axis.

```
jimenez.groupby('teamID').plot(x='yearID', y='Krate', grid=False)
```

![unknown](https://f.cloud.github.com/assets/81476/2089688/8a5c40c6-8e8a-11e3-8fc0-8e276ab7e2eb.png)

I have confirmed that `yearID` is `int64`.

Running Pandas '0.13.0-395-gef55e60' on OS X 10.9.1 (Python 2.7.5).
