This example displays a choropleth map of Swedish counties using the Mercator projection, with each county filled in blue and interior boundaries outlined in gray. The map renders as SVG through React, with geographic features generated from TopoJSON data using `d3.geoPath` and `d3.geoMercator`. The code applies `projection.fitExtent` to scale the map to fit the viewport, and county boundaries are extracted via `topojson.feature` and `topojson.mesh`. The visualization uses the d3 v5 library alongside React 16.9.