A circle follows the mouse across a full-window SVG. The visualization uses D3 v7’s `select` to append an SVG and a single circle, then listens to `mousemove` events on the SVG to update the circle’s `cx` and `cy` attributes. The page is styled with CSS to remove margins and hide scrollbars. No data source is involved; the piece demonstrates direct mouse-driven DOM attribute updates with D3’s event handling.