This example demonstrates a mouse follower where a circle visually tracks the cursor’s position across the viewport, serving as a minimal illustration of unidirectional data flow in a React-driven D3 visualization. The code uses D3’s `select` and `join` API to incrementally build an SVG and circle element, updating their `cx` and `cy` attributes based on mouse events. State changes trigger a re-render through the `setState` callback, with the initial position defaulting to the center of the window.