This example shows a three-level biological classification hierarchy, from a root “Living Things” node down to main categories like Animals, Plants, and Fungi, and then to leaf nodes such as Mammals and Flowering. The D3.js (v7) code uses `d3.hierarchy` and `d3.tree` to compute the layout, with SVG links drawn via `d3.linkHorizontal` and node shapes rendered as rectangles, circles, and rotated diamonds. Text labels and a legend are also included, all rendered as SVG elements with styling defined in the HTML.