This tree map visualizes hotel booking lead times aggregated by country, with each country’s total lead time represented as a rectangular area. The code uses D3 v5’s `nest()` and `rollup()` to sum lead times per country, then builds a hierarchy with `d3.hierarchy()` and lays it out with `d3.treemap()`. Data comes from a local `population.csv` file, rendered as an SVG canvas sized to the browser viewport. The example is built with React and D3, though the visible rendering relies solely on SVG and D3’s treemap layout.