A Tokyo choropleth map colors the city’s 23 wards according to their 2020 population, using a blue threshold scale with six bins from under 200,000 to over one million. The map’s SVG paths are drawn with `d3.geoPath` and a Mercator projection, while the color encoding relies on `d3.scaleThreshold` and `d3.schemeBlues`. A hover tooltip displays each ward’s name and population, and a horizontal legend from `d3-legend` labels the population ranges. The data comes from a GeoJSON file of ward boundaries and a CSV of population figures.