This example animates a double pendulum system, tracing the motion of two connected masses as they swing around a fixed center point. The code uses `d3.select` to create SVG circles and lines, updating their positions every frame with `setInterval` and `range(1)` to compute the pendulum’s coordinates. It relies on D3 v6’s data joins (`selectAll` and `join`) to render the moving balls, connecting rods, and shadow markers, with an animation loop that advances the angle `t` and the ratio `psi`.