This interactive collapsible tree diagram displays a hierarchical dataset with parent nodes that can be clicked to collapse or expand their child branches. The visualization uses d3.tree() to compute node positions, d3.hierarchy() to structure the data, and SVG paths drawn with a custom diagonal function for the linking curves. Nodes are rendered as circles with text labels, and transitions animate with 750-millisecond duration when the tree structure changes. The example derives from Mike Bostock's collapsible tree and uses d3.v5's selection, transition, and merge APIs for smooth interactive updates.