Two blue circles fade out over two seconds, then are removed and replaced by four red circles that fade in at new positions. The example demonstrates a sequential data-update pattern using SVG and D3 v7 transitions. The initial `circ` selection applies a `style("fill-opacity", "0")` transition with its `.on("end", ...)` callback, which calls `.remove()` on all circles before binding the second dataset and entering new circles with a reversed fade-in transition.