This visualization displays population density per square mile across U.S. census block groups using a choropleth map. A threshold color scale with d3.schemeOrRd maps density values to sequential red tones, while a legend with a sqrt scale and axis shows the breakpoints. The code loads TopoJSON data via d3.json, renders block group polygons with d3.geoPath, and overlays county boundaries as black outlines. An interactive legend is constructed with rect elements and d3.axisBottom.