fresh block
A choropleth map of U.S. county unemployment rates colors each county with a blue gradient from a threshold scale, using `d3.geoPath` to render TopoJSON boundaries. The visualization includes a horizontal legend with a linear axis, showing rates from 1% to 10%, plus white state borders. Unemployment data loads from a TSV file and matches each county by ID. The map uses `d3.scaleThreshold`, `d3.schemeBlues[9]`, and `d3.axisBottom`.