A vertical column of 50 circles animates across an SVG canvas, cycling horizontally in a loop. The circles are colored along a dark-to-light scale using `d3.scaleLinear` with HSL interpolation, creating subtle banding. D3 v4 selects the SVG, appends the circles, and chains `transition` calls with staggered `delay` and recursive `on("start", repeat)` to move each circle from left to right and back, demonstrating fundamental DOM manipulation and animation.