A tree diagram shows the hierarchical structure of global health data, with countries (Brazil, India, Australia, Nigeria) branching into disease categories and specific diseases. The visualization uses a horizontal tree layout with colored nodes—orange for countries, light blue for disease categories, and green for diseases—to distinguish hierarchy levels. The code employs D3 v7's `d3.hierarchy` and `d3.tree` APIs to compute layout positions, and renders the SVG with `linkHorizontal` paths and circles via a `.join()` operation. A legend above the chart explains the color coding.