This example displays a continuously rotating rainbow ring on a canvas, with a white gap that stretches and shrinks as the animation cycles. The code uses d3.arc with a canvas 2D context to draw 181 colored segments, each filled and stroked with d3.interpolateRainbow. A d3.timer drives the rotation by updating an angle offset, while d3.select transition with a custom tween on "angle" alternately stretches the start or end edge of the gap, creating a rhythmic pulsing effect.