This interactive scatter plot displays London’s monthly average air quality readings for PM2.5 and PM10 particulates, with each point colored by year. The visualization uses two dropdown menus implemented via the `dropdownMenu` function to let users switch the x- and y-axis variables among eight roadside and background pollutant metrics. Built with D3 v5, the chart renders SVG circles, uses `scaleLinear` for axes, `scaleOrdinal` with `schemeSpectral` for the year-based color scale, and supports a clickable color legend that highlights data points from a selected year.