This is a collapsible tree diagram built with D3 v6 that displays hierarchical data in a left-to-right layout. The example shows both a simple flat dataset and a larger flare JSON dataset, with nodes rendered as circles and curved links drawn as cubic Bézier paths. The code uses `d3.stratify` to convert flat parent-child pairs into a hierarchy, then applies a `d3.tree` layout to position nodes. SVG is used for rendering, with text labels positioned beside each circle based on whether the node has children.