This example shows an animated, collapsible tree diagram built with D3.js v3. The visualization presents a hierarchical dataset of fictional nodes and sizes, with the root node branching into two children and deeper sub-nodes. Users can click any node to collapse or expand its children, with smooth transitions adjusting node positions and links. The code uses `d3.layout.tree`, `d3.svg.diagonal`, and SVG transitions, with circles and text labels rendered for each node and gray paths connecting them.