A Multi-level drill-down donut chart in D3 v4. Expanded off code from https://codepen.io/ademps/pen/mlHyC but moves to v4 (vs v3), fixes some bugs (regarding outer 'parent' ring), and allows for infinite drill-down levels (vs 3 hard coded levels).
Demo: https://codepen.io/danbrellis/pen/aEMGMp
