This example demonstrates a D3 pack layout, which visualizes hierarchical data as nested circles. The dataset defines a tree with a root node, two levels of children, and leaf nodes that carry numeric values. Using `d3.hierarchy` and `d3.pack`, the code computes circle positions and radii so that parent circles encompass their children proportionally to summed values. The visualization renders each node as a semi-transparent circle with a white stroke, and labels only the leaf nodes. The SVG is dynamically sized to fit the container, and nodes are positioned via CSS transform translations.