This example shows a force-directed layout of 200 colored circles, where one invisible fixed root node is dragged by the mouse, causing the surrounding circles to collide and repulse around it. As the mouse moves, the root node's position updates, and the simulation resumes, with a quadtree-based collision detection preventing overlaps between the circles. The visualization uses the D3 v3 force layout, `d3.geom.quadtree` for collision handling, and SVG circles, with colors assigned via `d3.scale.category10`.