Add Grouped Bar Chart

[TaiWilkin]

Overview

Adds the bar chart for grouped responses.

Connects #22

Demo

Many countries

Few countries

Notes

The bars auto-resize to fit the height of the container. We could probably calculate a larger height for the container based on the number of countries/regions included if we'd rather the container grow instead of the items shrink.

Testing Instructions

• Select a number of countries or regions
• Select a question with a bolded answer (ie 'Head of Household')
• The chart should render the data correctly
• The chart should show all selected countries

[TaiWilkin]

@mmcfarland Unfortunately, I don't think there's any available way to align the order of the stacks. For some reason, the legend items are explicitly reversed when you use keys, and you can't provide separate keys for the legend and the bar chart. It appears that this was fixed for non-canvas bar charts, so maybe posting an issue to the library would help with this one.