A single gray circle tracks the mouse cursor as it moves across a blank white SVG canvas. The circle’s center directly follows the pointer position, which is captured and stored in React state on every mouse-move event. This example uses React’s `useState` and `useCallback` hooks, along with `d3.v7` for SVG rendering, to update the circle’s `cx` and `cy` attributes in real time. No data source is involved; the visualization simply demonstrates interactive pointer tracking within a fixed 960×500 viewport.