A hierarchical tree graph visualizes relationships among three countries, their health categories, and specific diseases. It uses a static, hand-coded dataset with India, France, and Brazil, each branching into two categories like Infectious or Respiratory, which further split into two diseases each. The code uses the d3.hierarchy and d3.tree layouts to compute node positions, and d3.linkVertical for curved link paths. Rendered as an SVG, circles and labels are color-coded by depth—orange for country, light blue for category, and light green for disease—with a legend clarifying the mappings.