Choropleth Map with Interactive Filtering
A choropleth map of global economies uses color to encode income groups. Clicking a color legend item filters the map to show only that category, dimming all other countries. The map is rendered using D3’s `geoPath` with a natural Earth projection and a spectral ordinal color scale. Interactive filtering is implemented through a custom `colorLegend` with `onClick` handlers and opacity transitions. Data is loaded from the world-atlas TSV and TopoJSON files.