This choropleth map of Spain overlays bubble markers on each province to compare COVID-19 cases and vaccination rates. Circle size encodes case counts via `d3.scaleSqrt`, while fill color uses `d3.scaleSequential` with `d3.interpolateRdYlBu` for vaccination percentages. The map is rendered with `d3.geoMercator` and `d3.geoPath` from TopoJSON data, with interactive tooltips and legend components built from `d3.csv` data and custom SVG groups.