都道府県別 人口の推移
This visualization shows the population changes of Japan’s 47 prefectures over time, using a choropleth map colored by population value. A user selects a year from a dropdown menu, triggering a smooth two-second color transition that updates prefecture fills. The map is rendered with D3 v3’s `d3.geo.mercator` projection and `d3.geo.path`, with circles marking prefectural capitals. Population data comes from the Statistics Bureau of Japan, and map boundaries use TopoJSON. A color legend built with `d3.legend` clarifies the scale.