Fixes #572 -- add grouped bar chart to the example
Fixes #572 -- add grouped bar chart to the example

uwdata/vega-lite,uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,ellisonbg/vega-lite,vega/vega-lite,vega/vega-lite,vega/vega-lite,vega/vega-lite,uwdata/vega-lite,ellisonbg/vega-lite,uwdata/vega-lite,ellisonbg/vega-lite