This example demonstrates how d3-force can be used to group and ungroup data-driven circles through user interaction. Nine circles, colored by group (green for "one", orange for "two") and sized with a square-root scale, are simulated with forces for y-centering, centering, and collision avoidance. Clicking the SPLIT button swaps the x-force to push circles of group "one" toward x=200 and group "two" toward x=700, while JOIN returns all circles to the horizontal center. The simulation’s alphaTarget and restart methods smooth the transitions between the two layouts, and clicking a circle displays its name and value in the footer.