Choropleth Map with Interactive Filtering
A choropleth map displays countries colored by economic classification, with a legend of color swatches. Clicking a legend item filters the map to highlight only countries in that category, dimming all others, while hovering over a legend item reveals its effect. Built with D3 v5, the visualization uses `geoNaturalEarth1` and `geoPath` for the map projection and rendering, plus `scaleOrdinal` with `schemeSpectral` for colors. Geographic data comes from the World Atlas TopoJSON and TSV files.