A collapsible tree diagram visualizes hierarchical family relationships, rendering nodes and links as an animated SVG. The example uses D3.js version 3.5 with the `d3.layout.tree` and `d3.svg.diagonal` APIs to compute the layout and draw curved paths. Circles and text labels represent individuals, and clicking a node toggles its children’s visibility with smooth transitions. Styling is applied via external CSS, and the input data comes from a local `qn_tree.json` file.