This animated choropleth map visualizes changes in PM2.5 particle concentrations across U.S. counties from 2000 to 2018. Users can scrub through years via a vertical slider or trigger automatic playback, with color intensity reflecting pollution levels via a d3.interpolateYlGnBu sequential scale. Built with React and D3 v5, the SVG map uses d3.geoPath with a geoIdentity projection for county boundaries, while a ColorLegend component renders a gradient bar with tick marks. The data is loaded via d3.csv, and each state’s border is highlighted on hover.