This example shows a hierarchical tree visualization of COVID-19 data, with branches for continents, countries, and leaf nodes displaying cumulative confirmed cases, deaths, and recoveries. The graph uses a standard d3.tree layout with SVG rendering, where nodes are circles connected by curved links and labeled with country names. The code defines the data structure manually, applies d3.hierarchy to nest the data, and uses d3.tree().size() to compute node positions. Styling is applied through CSS classes for nodes and links, with the visualization rendered inside an SVG container.