A world choropleth map colors each country by its income group, using a sequential red color scale derived from World Bank data. The visualization renders with D3 v7 and the `geoNaturalEarth1` projection, building paths via `geoPath` and TopoJSON country geometries. A custom color legend animates its circles with a `transition` to reflect scale changes, while a zoom behavior lets users pan and scale the map. Country names appear as tooltips on hover, drawn from the accompanying TSV metadata.