Fifteen circles undulate in an animated sine-wave pattern, with positions and radii oscillating smoothly over time. The example demonstrates D3's selection and data-join mechanics by using `selectAll` and `join` to bind an array of generated data to SVG circle elements. Rendered in SVG, the animation updates every 100 milliseconds via `setInterval`, incrementing a phase variable to drive the motion. The code imports `select` and `range` from D3 v7, showing the modern `join` pattern as a concise replacement for the older `enter`/`merge` approach.