A single dark circle follows the mouse cursor across an SVG canvas in a React-based visualization. The circle’s position updates live via React’s `useState` hook, driven by the SVG `onMouseMove` event, which captures the cursor’s client coordinates. Rendering uses plain SVG elements, with D3 v5 loaded but not actively used in the logic. The example demonstrates reactive state-driven updates in React for interactive graphics, with the circle centered on the pointer’s exact location.