Two groups of circles swap places on a D3.js SVG stage: two blue circles fade out and are removed, then four red circles fade in. The example demonstrates a common sequencing pitfall—the new data join executes before the old elements’ removal transition completes, causing selection conflicts. It uses d3.selectAll, data joins, .enter(), and transition().remove() in D3 v7, with fill-opacity transitions timed via .delay() to coordinate the fade-out and fade-in phases.