This example shows a double pendulum animation, drawing a small ball as the endpoint of two connected, rotating line segments. The code uses d3.js v6 with a time-driven `setInterval` loop to update the pendulum's position based on a constant time step and a phase-shift ratio (`psi`). It renders the scene as SVG, creating the arm lines, a moving ball, and a large hollow circle marker at the endpoint. The visualization demonstrates the harmonic motion of the two-segment system.