This example shows a partial smiley face: a single thick arc forming the bottom half of a mouth, centered on an otherwise blank SVG canvas. The arc is drawn using `d3.arc()` with `innerRadius` and `outerRadius` set from `mouthRadius` and `mouthWidth`, and its angles span from −0.5π to 0.5π. The visualization is built with React 17 and D3 v7, rendering the shape via an SVG `path` element inside a translated group. No eyes or other facial features are present.