This choropleth map colors countries by income group using an ordinal color scale from the `d3.scaleOrdinal` and `d3.schemeSpectral` APIs. It projects the world with `d3.geoNaturalEarth1` and `d3.geoPath`, drawing each country as an SVG path. The map is interactive, supporting zoom and pan via the `d3.zoom` event handler, and includes a custom legend generated with `colourLegend`. Data combines TopoJSON country boundaries with TSV attributes from the World Atlas dataset.