A treemap layout visualizes a small hierarchical dataset, with nested rectangles representing the root node A1, its children B1 and B2, and the grandchildren C1, C2, and C3. Each rectangle is colored cadetblue, and labels show the node names. The example uses the `d3.treemap` layout with `d3.hierarchy` and `rootNode.sum` to compute sizes, then renders the result as SVG rectangles and text elements via data joins on `g` nodes.