This example shows a hierarchical tree diagram of programming languages, with a root node, three category branches for Frontend, Backend, and Data Science, and leaf nodes for individual languages. Each depth level uses distinct colors and shapes—circles for the root and leaves, rectangles for the middle layer—to visually separate the tree’s levels. The code builds the hierarchy with `d3.hierarchy()` and lays it out using `d3.tree()`, rendering links as curved paths via `d3.linkHorizontal()`. Node styling and labels are applied directly to SVG elements, with a legend added to clarify the level color coding.