A black circle follows the mouse cursor across an SVG canvas, serving as a playful interactive pointer. The example is built with React and D3 v7, using React state to track mouse coordinates via an `onMouseMove` event handler. The `useState` hook stores the current `clientX` and `clientY` positions, and a `<circle>` element updates its `cx` and `cy` attributes accordingly. The SVG's width and height are fixed, and the circle has a constant radius of 30 pixels.