Reference: https://observablehq.com/@project-essex/introduction-to-d3-and-svg

D3 V4 for circle packing： https://bl.ocks.org/denjn5/6d5ddd4226506d644bb20062fc60b53f

D3 V5 for circle packing: https://bl.ocks.org/HarryStevens/4fba7a62b0ff302ef49768198d4c54c6 https://observablehq.com/@johnhaldeman/tutorial-on-d3-basics-and-circle-packing-heirarchical-bubb

计时器： https://codepen.io/freeCodeCamp/pen/XpKrrW

See also React Starter.