A circle follows the mouse cursor across an SVG canvas, updating its position in real time. The visualization is built with React and D3 v6, using React's `useState` and `useCallback` hooks to track and update the mouse coordinates from the `onMouseMove` event. The circle is rendered as an SVG element with a fixed radius, and its `cx` and `cy` attributes are bound to the current mouse position. The example demonstrates a simple interactive pattern in a React-D3 integration.