D3 test
This example visualizes a historical route from a GeoJSON file, displaying daily segments as dashed black polylines over a light blue graticule-filled map. The code uses D3 v5 with an SVG rendering approach and a Mercator projection configured via `d3.geoMercator()`. It loads coastline and route data with `d3.json()`, then binds the features to SVG paths using `d3.geoPath()`. The graticule is generated with `d3.geoGraticule()`. The visualization clearly shows the path’s progression across the mapped region, with each day’s travel distinguished by styling properties like stroke dasharray.