d3js workshop - circles
Three circles are rendered in an SVG, with a red fill and a staggered transition that delays each circle’s color change and radius increase by one second per circle. The example demonstrates D3’s data join, binding an array of numbers to circle positions, and uses `transition().delay()` and `transition().duration()` to animate style and attribute changes over two seconds. The code explicitly appends an SVG element, selects circles via `d3.select`, and applies transitions to fill, radius, and vertical position, showing basic animation and data-driven updates with D3 v3.