Circles on a map
This example visualizes COVID-19 case counts by country on a natural Earth projection map, with circles sized by total cases as of May 16, 2020. Using d3.geoNaturalEarth1 and d3.geoPath, it renders country outlines and positions proportional circles via d3.geoCentroid. A d3.scaleSqrt maps case numbers to circle radii, and a custom size legend (with d3.format) explains the scaling. The map supports zooming through d3.zoom, while country data is joined from a CSV file and TopoJSON world-atlas geometry.