This interactive tree diagram visualizes hierarchical EV data by country, vehicle type, and parameter, with collapsible nodes that let users explore nested branches. Built with D3.js v6, the visualization uses `d3.hierarchy` and `d3.tree` to compute node positions, renders nodes as SVG circles with text labels, and draws curved links via path elements. Clicking a node triggers a smooth animated transition that expands or collapses its children, while branches beyond the second level start collapsed to reduce clutter. All rendering is SVG-based with CSS styling applied to node fills and cursor states.