This is a d3.js tree diagram that incldes an interactive element as used as an example in the book D3 Tips and Tricks v4.x.
Any parent node can be clicked on to collapse the portion of the tree below it, on itself. Conversly, it can be clicked on again to regrow.
It is derived from the Mike Bostock Collapsible tree example and updated to use v4.
Kudos and thanks also go out to Soumya Ranjan for steering me in the fight direction for the diagonal solution.