Re-factor chord diagram to use d3.layer
Re-factor chord diagram to use d3.layer

Implement minimal changes to take advantage of the newly-introduced
d3.Layer framework.

jugglinmike/d3.chart,misoproject/d3.chart,jugglinmike/d3-experiments,tbranyen/d3.chart,tbranyen/d3.chart,misoproject/d3.chart