Choropleth Map
A choropleth map visualizes the global prevalence of HIV/AIDS among adults aged 15-49 in 2017. Countries are filled with a yellow-orange-red sequential color scale from `d3.scaleSequential` and `interpolateYlOrRd`, with gray for missing data. The map uses `d3.geoNaturalEarth1` projection and `d3.geoPath` on TopoJSON data from the World Atlas, rendering country boundaries and graticules via SVG. HIV prevalence data comes from IHME, joined to country codes via ISO-3166 reference data.