This choropleth map colors world countries by income group, using a spectral color scale with a legend showing the categorical ranges. It renders country boundaries as SVG paths via the `d3.geoPath` and `d3.geoNaturalEarth1` projections, with a zoom interaction applied through `d3.zoom`. Data comes from the `world-atlas` TopoJSON and TSV files, processed with `topojson.feature` to merge attributes. Hovering highlights a country in red and displays its name and income group in a tooltip.