The visualization shows a collapsible tree layout of the Santander dataset, with an interactive focus on hierarchy exploration. Built with D3 v3, the tree uses SVG and animated transitions to expand and collapse child nodes when clicked. The data is loaded from a local JSON file and maps municipal information from the Santander region, organized into urban and rural categories. The tree layout is positioned with margins and uses diagonal links, while nodes are rendered as white circles with steelblue strokes. Initially, all nodes are collapsed to their top-level categories; clicking a node toggles its children, animating the tree’s expansion and contraction with a 750ms duration. The root is centered vertically, with each depth level offset horizontally, and the visualization is scaled to fit an 800px tall frame.