This example shows a minimalist loading spinner rendered as a single animated arc that rotates continuously around a circular track on a pale green background. The animation uses D3 transitions with a 20-second duration, where `transition.attrTween` interpolates the arc’s `endAngle` from 0 to 2π, and an `on("end")` callback restarts the spin indefinitely. The code builds the shape with `d3.arc()`, setting outer and inner radii relative to the container size, and positions it via an SVG `<g>` transform.