This example demonstrates D3’s data join and update pattern through an animated swarm of purple circles. It shows how circular marks continuously reposition and resize based on a periodic mathematical function. The visualization uses D3 v7’s `select` and `range` to generate data, appends an SVG to the body, and updates circles every 100/60 milliseconds via `setInterval`. The `.join('circle')` method handles enter and update selections, binding new data arrays while styling radius, x/y coordinates, and fill.