A fork of a three-layer D3 v7 tree visualization renders a hierarchical dataset of continents, countries, and cities using SVG. The code builds a vertical tree layout with `d3.hierarchy` and `d3.tree()`, then draws curved links with `d3.linkVertical()`. Node shapes encode depth: rounded rectangles for the root and countries, diamonds for continents, and circles for leaf cities, with a legend explaining each.