A tree diagram displays a three-level programming language hierarchy, from a root node to category nodes for Frontend, Backend, and Data Science, each with leaf-node languages. The visualization uses D3.js v7 to build the SVG layout, with `d3.hierarchy` and `d3.tree` computing positions. Node shapes encode type: rectangles for root, circles for categories, and rotated squares for leaves, all connected by `d3.linkHorizontal` paths.