This example shows a set of rectangles that expand and contract when clicked, revealing hidden nested content. Built with D3 v3 and rendered as animated SVG, the visualization uses custom helper functions from d3-jetpack to simplify attribute handling and transitions. Rectangles are likely bound to hierarchical data, with expansion triggered by mouse events to reveal child nodes or additional details. The animation is driven by D3's transition system, and the layout adapts to the data's depth. The code relies on a lightweight d3 starter kit for scales and axes, while the jetpack library provides syntactic sugar for concise, readable D3 code.