This example shows a world map colored by fertility rate per country, using a blue-to-white linear scale to indicate values. The map is built with D3 v4, loading GeoJSON topology from a GitHub repository and CSV data from another source, then joining them by country name. It uses `d3.geoMercator` for projection and `d3.geoPath` to draw country boundaries as SVG paths. The color scale is created with `d3.scaleLinear` and applied to each country’s fill, rendering a choropleth visualization in a static SVG canvas.