A collapsible tree diagram visualizes a hierarchical file system and music directory, with root "Directory" branching into "Document," "Pictures," "Music," and "Rock." The example uses D3.js version 3.5's `d3.layout.tree` and `d3.svg.diagonal` to render purple node-link SVG diagrams, with an animated transition on update. The data is hardcoded as a JavaScript array, with interactive node clicking enabled.