This method is used to set a sort order for the groups.
The diagram on the left uses the default sort function which is d3.ascending
and the diagram on the right uses a custom sort function.
Modified http://vizjs.org/viz.v1.1.0.min.js to a secure url
https://d3js.org/d3.v4.min.js
https://vizjs.org/viz.v1.1.0.min.js