This choropleth map of U.S. counties uses a quantile color scale to display air quality index (AQI) values, with gray indicating missing data. A histogram below the map shows monthly mean AQI values over time. Users can brush over the histogram to filter the map to counties within a selected time range. The visualization combines React with D3 APIs including geoAlbers, geoPath, scaleQuantile, brushX, and histogram, rendering to SVG. Data is loaded from CSV and TopoJSON sources.