This horizontal tree diagram shows a three-level hierarchy with a root node, two second-level nodes, and two leaf nodes, rendered as white circles with steel-blue outlines connected by curved gray links. Using d3.v7’s `d3.tree` layout and `d3.hierarchy` functions, the code positions nodes along the x-axis and assigns coordinates via the `treemap` call, then draws SVG paths for the links and SVG groups for the nodes, with labels placed to the left or right of each circle depending on whether the node has children. The data is hardcoded inline as a JSON object, and the diagram is built entirely with SVG elements appended to the page body using d3.select and `.append`.