These county, state and country boundaries are extracted from a single TopoJSON file from us-atlas. Counties are stroked in thin gray, states in thin black, and the country in thick black. See also the fancy drop shadow variant.
forked from mbostock's block: U.S. TopoJSON
https://d3js.org/d3.v4.min.js
https://d3js.org/d3-scale-chromatic.v1.min.js
https://d3js.org/topojson.v1.min.js