This visualization displays daily Air Quality Index (AQI) readings across the United States in 2021, combining a geographic scatterplot with a synchronized time-series histogram. Users can brush over the histogram to filter the map’s colored circles by date, with color intensity reflecting AQI levels. The map uses `d3.geoAlbersUsa` and `d3.geoPath` to render state boundaries, while the histogram employs `d3.bin`, `d3.scaleTime`, and `d3.brushX` for interactive selection. Data is loaded via `d3.csv` from a GitHub-hosted dataset, and React manages component state for the brushing extent.