A hierarchical tree diagram displays brand-model relationships. The visualization shows a root "Brands" node branching into brand categories like OnePlus and Samsung, which further split into individual model names. The code uses `d3.rollup` to aggregate sample data into the tree structure, then applies `d3.tree()` and `d3.hierarchy()` to compute node positions. `d3.linkHorizontal()` draws curved links between nodes, while SVG circles and text labels render the hierarchy.