This example demonstrates a single red circle that follows the mouse cursor across the visualization canvas. The circle's position updates directly in response to pointer movement, with the cursor’s coordinates captured from each mouse event. The implementation uses d3’s `select` and `pointer` functions within an interactive SVG, where a `mousemove` handler triggers state updates via a spread operator to maintain an immutable state object. The circle’s `cx` and `cy` attributes are then re-rendered from the updated state, showcasing a simple, reactive update pattern.