A circle follows the mouse cursor across a full-viewport SVG. The example demonstrates unidirectional data flow: mouse events trigger a state update, which then re-renders the visualization. Built with D3 v7, it uses `select`, `join`, and event handlers to bind the circle’s `cx` and `cy` attributes to the current cursor position. The state is managed externally and passed into the `viz` function, with `setState` triggering a re-render on each mouse move. No external data source is used; the interaction is purely based on user input.