Question

Furthermore, each node must have one attribute:

• parent - the parent node.

This is a subset of the fields generated by the hierarchy layouts. The return value of the layout is an array of paths, where each path is represented as an array of nodes. Thus, the bundle layout does not compute the basis splines directly; instead, it returns an array of nodes which implicitly represent the control points of the spline. You can use this array in conjunction with d3.svg.line or d3.svg.line.radial to generate the splines themselves. For example, if you were to use a cluster:

var cluster = d3.layout.cluster()
    .size([2 * Math.PI, 500]);

A suitable line generator for hierarchical edge bundling might be:

var line = d3.svg.line.radial()
    .interpolate("bundle")
    .tension(.85)
    .radius(function(d) { return d.y; })
    .angle(function(d) { return d.x; });

The bundle layout is designed to work in conjunction with the line generator's "bundle" interpolation mode, though technically speaking you can use any interpolator or shape generator. Holten's bundle strength parameter is exposed as the line's tension.