This choropleth map displays county-level ozone concentrations across the United States from 2000 to 2021. A vertical slider and play button animate through years, with the sequential YlGnBu color scale mapping ozone levels while gray indicates missing data. Built with React and D3 v5, it uses `geoPath` with an Albers USA projection and TopoJSON county/state boundaries, rendering SVG paths with hover tooltips showing county names and values.