Chained Transitions

This animation demonstrates the use of transition.transition and d3.active to create chained transitions. Chained transitions inherit the reference time of the original transition, guaranteeing synchronicity.

<!DOCTYPE html>
<meta charset="utf-8">
<style>

circle {
  fill: #000;
  stroke: #000;
  stroke-width: 1.5px;
}

</style>
<svg width="960" height="500"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>

var svg = d3.select("svg"),
    margin = {top: 40, right: 40, bottom: 40, left: 40},
    width = svg.attr("width") - margin.left - margin.right,
    height = svg.attr("height") - margin.top - margin.bottom,
    g = svg.append("g").attr("transform", "translate(" + margin.left + "," + margin.top + ")");

var y = d3.scalePoint()
    .domain(d3.range(50))
    .range([0, height]);

var z = d3.scaleLinear()
    .domain([10, 0])
    .range(["hsl(62,100%,90%)", "hsl(228,30%,20%)"])
    .interpolate(d3.interpolateHcl);

g.selectAll("circle")
  .data(y.domain())
  .enter().append("circle")
    .attr("r", 25)
    .attr("cx", 0)
    .attr("cy", y)
    .style("fill", function(d) { return z(Math.abs(d % 20 - 10)); })
  .transition()
    .duration(2500)
    .delay(function(d) { return d * 40; })
    .on("start", function repeat() {
        d3.active(this)
            .attr("cx", width)
          .transition()
            .attr("cx", 0)
          .transition()
            .on("start", repeat);
      });

</script>

https://d3js.org/d3.v4.min.js