This example displays a hierarchical tree of world countries organized by subregions under an overall "Grades" root node. The visualization renders country names as text labels connected by curved horizontal links, with font size decreasing by depth level. It uses D3 v5’s `tree` layout, `hierarchy`, and `linkHorizontal` generators to compute positions and paths, while the SVG output supports pan and zoom via `d3.zoom`. Data comes from a static JSON file.