This choropleth map colors world countries by population, using darker blues for more populous nations while unlisted countries appear gray. A legend explains the visual encoding. The example loads world-atlas TopoJSON, converts it to GeoJSON features using topojson-client, and projects the map with d3.geoNaturalEarth1 and d3.geoPath. A d3.scaleLinear maps population values to a light-to-dark color range, and SVG paths render each country with tooltips showing population figures.