d3.js v4: How to access parent group's datum index?

Question

The description of the selection.data function includes an example with multiple groups (link) where a two-dimensional array is turned into an HTML table.

Answer 1

I ended up defining an external variable "j" and then increment it whenever "i" is 0

example V3 snippet below.

rowcols.enter().append("rect")
 .attr("x", function (d, i, j) { return CalcXPos(d, j); })
 .attr("fill", function (d, i, j) { return GetColor(d, j); })

and in V4, code converted as below.

var j = -1;
rowcols.enter().append("rect") 
 .attr("x", function (d, i) { if (i == 0) { j++ }; return CalcXPos(d, j); })
 .attr("fill", function (d, i) { return GetColor(d, j); })