This example displays a single black circle that follows the mouse cursor across an SVG canvas. The circle’s position updates live based on the cursor’s coordinates, which are tracked through React state. The code uses React’s `useState` and `useCallback` hooks, an SVG `<circle>` element, and D3.js version 5 for the underlying library. The rendering relies on ReactDOM to mount the component, with no additional data sources or animations beyond the direct mouse-following interaction.