A choropleth map of population density per square mile colors U.S. census block groups using a threshold scale and the sequential color scheme `d3.schemeOrRd[9]`. The visualization includes a square-root-scaled legend with an axis, and overlays county boundaries as black strokes at 30% opacity. The code loads TopoJSON data via `d3.json`, converts it with `topojson.feature`, and renders it with `d3.geoPath`. The legend uses `d3.scaleThreshold` and `d3.axisBottom`. The map is built with D3 v4 and React in the supporting files.