A single black circle follows the mouse cursor across an SVG canvas, with its position updating reactively in real time. The circle’s center is driven by the `clientX` and `clientY` coordinates from the SVG element’s `onMouseMove` event, stored in React state via `useState` and `useCallback`. The visualization uses React 17 for rendering, D3 v7 for SVG utilities, and demonstrates a simple interactive pattern without any data source or external dataset. The circle radius is fixed at 30 pixels, and the SVG viewport is 960 by 500 pixels.