A tree graph displays hierarchical data as connected nodes branching from a single root. The visualization uses a force-directed layout via `d3.forceSimulation` to position nodes, with links drawn as curved paths. Nodes are rendered as circles colored by depth, and the graph updates smoothly through D3’s `transition` when re-simulated. The code loads a JSON dataset and relies on `d3.hierarchy` and `d3.tree` to compute the tree structure before applying the force simulation for final placement.