Radial Tree of Countries Hierarchy
This radial tree visualization displays the hierarchical relationship of world countries, organized by continent and subregion, with the root node "World" at the center. The layout uses D3's `d3.cluster` to position nodes radially, and SVG paths connect parent and child nodes with curved links. Text labels show country and region names, with font sizes scaled according to depth using `d3.scaleSqrt`. Users can pan and zoom the entire visualization via `d3.zoom`, which applies an SVG transform to the containing group. Data is loaded from a local JSON file.