It might be helpful to let reader know that we are going to hold the selected region entries of the legend as a set of values in the selectionModel -- here upfront. Alternatively it might be easier to follow if instead of defining a general purpose selectionmodel, a selectionModel specific to the regions in the map was defined. Could then later abstract the design pattern into a general-purpose SelectionModel.

This cell is where I get confused as it is difficult to see the connection between region.toggle() and the toggle function defined in SelectionModel() because I haven't been told that region=SelectionModel(). I might add that in before talking about the region.toggle.

Made some updates to both the text and legend code below to further clarify.

region.on('change.chart', ...) should be region.on('change.legend', ...) according to text above I believe.

No, change.legend is for the legend's internal listener. This is a different listener, for the main chart. If we named if change.legend it would override the legend's listener due to how d3-dispatch is designed... I will add some more comments in the code to clarify.