A single purple circle follows the cursor within an SVG canvas, reacting in real time to mouse movement. The example demonstrates React state management for interactive visualization: the `useState` hook stores the cursor's x and y coordinates, while the `useCallback` hook memoizes the mouse event handler for performance. The SVG `<circle>` element updates its center position based on the tracked coordinates, with d3.v5 loaded for potential enhancements. React 16 renders the component, and the circle is initially centered before the first mouse move.