An animated scatter of sine-driven circles demonstrates D3’s data join with `selection.join`. The number of circles oscillates as the array length changes, while each circle’s x-position increments by 50 pixels and the y-position follows a sine wave. The SVG viewport resizes to the window, and the animation updates every 10 milliseconds. Using `d3.range` and `Math.sin`, the code smoothly re-enters, updates, and exits circles, showcasing D3 v3’s reactive data binding and the concise `join` API.