This choropleth map colors countries by their income group using a categorical color scale applied from the `schemeSpectral` range. The visualization renders geographic boundaries from TopoJSON data processed with `topojson.feature` and projects them using `d3.geoNaturalEarth1` and `d3.geoPath`. A legend, built with a custom `colorLegend` function, displays the colored circles and labels for each income category. Country names and income groups appear as tooltips via SVG `title` elements, and the map supports zooming through `d3.zoom`.