This example shows a yellow circle that follows the mouse cursor within an SVG canvas. The circle’s position updates reactively as the user moves the mouse over the 1000×500 viewport. Built with React 16.9 and D3 v5, it uses React’s `useState` and `useCallback` hooks to track mouse coordinates and re-render the circle’s `cx` and `cy` attributes. The SVG element’s `onMouseMove` event triggers state updates, demonstrating a simple interaction pattern without any external data source.