A choropleth map of Massachusetts displays town boundaries colored by county, with an interactive legend that highlights towns in the selected county on mouse hover. Built with React and D3 v5, it uses `d3.geoAlbers` projection with custom rotation and scale via `d3.geoPath` for rendering SVG paths. The legend, implemented with HTML circles and text, updates a hovered value state to filter which towns receive the highlight. Town-county mappings come from a CSV, with geoJSON spatial data from a gist.