A collapsible tree of world countries, arranged hierarchically, fills a rounded-rectangle frame. The visualization renders an interactive node-link diagram where parent nodes expand to reveal child countries. Built with D3 v5, the example uses an SVG container and a force-directed layout via `d3.forceSimulation` to position the nodes. The tree structure is defined by the data source, and the code selects the SVG element and sets its dimensions programmatically, with styling handled by an external CSS file.