This example visualizes an isochrone map, showing a blue, semi-transparent polygon that represents the area within a 5,000-meter radius of a central point in East Grinstead, UK. The polygon is generated by creating a grid of candidate points and filtering them based on distance. The code uses the Turf.js library for distance calculations and convex hull generation, and renders the result with D3’s `d3.geoMercator` projection and `d3.geoPath`.