A choropleth map of the United States displays county-level AQI values, colored with a quantile scale across a green-to-red gradient, with gray for missing data. A date histogram below (though not rendered in the visible SVG) is intended to show AQI trends over time. Built with React and D3 v5, the map uses `d3.geoAlbers` and `d3.geoPath` to draw county boundaries from TopoJSON, while `d3.csv` loads the air quality dataset and `d3.scaleQuantile` assigns colors.