Grouped Color Bar Charts

[gavinhodge]

It would be nice to do grouped color bar charts, showing multiple categories per tick mark.

Here's a rather advanced example: http://bl.ocks.org/mbostock/3887051

[johanndt]

I'd just like to report back that we've been using the grouped_bars function from this notebook for more than 6 months in production without a single issue ever coming up. Even just merging that function into the repo would be sufficient for most I'd say:

https://github.com/sandialabs/toyplot/blob/4a91634a0c7ebed068772010813b97c2e91f7fa7/design/grouped-bars.ipynb