ValueError on groupby boxplot
I have a pretty simple data frame of two columns, containing dates and velocities

```
    velocity    date
0    83.9    2007-08-04
1    89.6    2007-08-04
2    88.2    2007-08-04
3    88.9    2007-08-04
4    89.4    2007-08-04
```

Yet, when I try to make a box plot of velocities grouped by date, I get a ValueError:

```
pitches.boxplot(column='velocity', by='date')

ValueError: Wrong number of items passed 109, indices imply 108 
```

I've confirmed that `velocity` are floats and `date` are np.datetime64.
