Color bars by category
Would it make sense to add functionality for coloring bar charts according to a categorical value in a column? It could then by be passed as an argument such as:

``` python
df.plot(kind='bar', colors=['r', 'g', 'b'], color_by=['B'])
```

which would lead to ['r', 'g', 'b'] being cycled throughout the values in column 'B'

Something along the lines of:

``` python
def color_by_category(series, colormap):
        from itertools import cycle
        unique_categories = set(series)
        color_cycle = cycle(colormap)
        repeat_colors = [color_cycle.next() for i in range(len(unique_categories))]
        category_color = {i:j for i,j in zip(unique_categories, repeat_colors)}
        color_array = [category_color[k] for k in series]

        return color_array
```
