A React application visualizes air quality index (AQI) trends over time for a selected city, filtered via a dropdown menu. The chart displays a natural curve fitted to AQI readings with labeled points, using an SVG rendering. It leverages D3 v7 utilities, including `scaleTime`, `scaleLinear`, `line` with `curveNatural`, and `timeFormat` for axes, and loads its CSV dataset directly in code.