A choropleth map displays global income groups, coloring each country through a categorical `scaleOrdinal` with the `schemeSpectral` range. Users click legend items to filter the map, highlighting matching countries while dimming others and the sphere backdrop. The map renders via `geoNaturalEarth1`, `geoPath`, and `zoom` interactions, while the legend uses click handlers and opacity toggling. Data comes from TopoJSON world-atlas and TSV files loaded through `d3.json` and `d3.tsv`.