This example displays a centered cyan circle on a full-screen SVG. It demonstrates the minimal setup for a D3 visualization within a React environment, using `d3.select` to append the `<svg>` element and then attaching the circle with `svg.append`. The circle’s position is set to the center of the viewport using `innerWidth` and `innerHeight`, and its fill is specified via the `attr` method. The code relies on the D3 library (version 7.8.5) and the `select` API, with the rendering handled entirely by SVG.