This tree diagram visualizes a small, three-level dataset of animals, showing the hierarchical structure from the root "Animals" through categories like "Mammals" and "Birds" down to individual species. Nodes are drawn as colored circles, with a distinct color per depth level, connected by horizontal links. The example uses D3 v7 APIs including `d3.tree`, `d3.hierarchy`, `d3.linkHorizontal`, and `d3.scaleOrdinal`, with SVG rendering for all elements.