implement data threshold grouping

[davelandry]

d3plus 1.0 has a feature that groups data under a certain percentage ("threshold") into an "Values < X%" category. This should be implemented in the new d3plus:

• a threshold method that accepts a Number or an accessor Function
• a thresholdKey method that tells d3plus-viz what value to use (accepts a String or a Function). It will then be on each visualization (ie. Treemap) to set a default value for this method (ie. when a user defines sum, also define thresholdKey if it has not been defined.
• grouping logic should be implemented after this line
• if it is a Function, pass the function the flatData and expect a returned Number
• add a key to the nest that nests any data that has a thresholdKey less than that the overall total for that group and give it a different key

[frabarz]

I need a clarification on the way the new category would be made. The "Values < X%" makes me think on a "all items whose d[value] is lower than sum(d[value]) * threshold" criteria, but Carlos made me think it would also useful to group "all items belonging to the lower threshold * 100% of the data".

[davelandry]

from the Slack conversation: let's make it "all items whose value is lower than the overallTotal * threshold, where overallTotal is calculated across the entire dataset (not for each group)

[davelandry]

implemented!