This example displays a hierarchical tree graph of cities organized by their parent countries, with each country as a root and cities as leaf nodes. It shows the structure of the data using circular nodes connected by curved, bezier path links. The code uses d3.stratify to convert the flat array of city-country pairs into a hierarchy, then d3.tree to compute the layout and position nodes. Node circles and text labels are rendered as SVG elements, with links drawn using custom path commands that create smooth curves between parent and child nodes.