Continuous expanding circle ping

<!DOCTYPE html>
<meta charset="utf-8">
<style>
	svg {
		background-color: #3b3b3c;
	}
	circle {
		fill: none;
		stroke: #398FBD;
	}
</style>

<body>
	<script src="https://d3js.org/d3.v3.min.js"></script>
	<script>
		var margin = {
			top: 40,
			right: 40,
			bottom: 40,
			left: 40
		}, width = 960 - margin.left - margin.right,
			height = 500 - margin.top - margin.bottom;

var y = d3.scale.ordinal().domain(d3.range(1)).rangePoints([0, height]);

var svg = d3.select("body")
			.append("svg")
			.attr("width", width + margin.left + margin.right)
			.attr("height", height + margin.top + margin.bottom)
			.append("g").attr("transform", "translate(" + margin.left + "," + margin.top + ")");
		svg.selectAll("circle")
			.data(y.domain())
			.enter()
			.append("circle")
			.attr("stroke-width", 20)
			.attr("r", 10)
			.attr("cx", width / 2)
			.attr("cy", y)
			.each(expand);

function expand() {
			var circle = svg.select("circle");
			(function repeat() {
				circle = circle.transition()
          .transition()
          .duration(2000)
          .attr('stroke-width', 0)
          .attr("r", 250)
          .ease('sine')
          .transition()
          .duration(0)
          .attr('stroke-width', 15)
          .attr("r", 15)
          .ease('bounce')
          .each("end", repeat);
			})();
		}
	</script>

Modified http://d3js.org/d3.v3.min.js to a secure url

https://d3js.org/d3.v3.min.js