A compact tree diagram visualizes a four-level hierarchy: the root node "World" branches into continents, which split into countries. The code defines custom node shapes—a blue rounded rectangle for the root, purple circles for continents, and green ellipses for countries—with each node labeled. Using the d3.hierarchy and d3.tree layouts with a vertical link generator, the graph is centered horizontally on the page. The visualization is rendered as SVG with distinct colors per level, sourced from a static inline JavaScript data structure.