This example demonstrates mouse tracking by rendering a single circle that follows the cursor across an SVG canvas. It uses React state and a `handleMouseMove` callback to capture the pointer’s `clientX` and `clientY` coordinates, updating the circle’s center position with each movement. The visualization renders with D3 v5 and React 16.9, drawing the circle using SVG elements with a white fill, black stroke, and fixed radius of 60 pixels.