Legend Component Mock Up

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>

<body>
  <script>
    
    var randvals = [10,20,50];
    
    var width = 200;
    var pad = 40;

var radiusScale = d3.scaleLinear()
    	.domain([Math.sqrt(5),Math.sqrt(50)])
    	.range([2,width/2 - pad]);
    
    // Feel free to change or delete any of the code you see in this editor!
    var svg = d3.select("body").append("svg")
      .attr("width", width)
      .attr("height", width)
		
    svg.append("text")
    	.attr("dx",width/2)
    	.attr("dy",pad/2)
    	.style("font-family","Franklin Gothic Medium")
	    .style("text-anchor","middle")
    	.text('Legend')
    
    svg.selectAll("circle")
    	.data(randvals)
    	.enter()
    	.append("circle")
    	.attr("r",(d)=>{return radiusScale(Math.sqrt(d))})
      .attr("fill","none")
    	.attr("stroke","rgba(0,0,0,.5)")
      .attr("cy", (d)=>{return width - pad - radiusScale(Math.sqrt(d))})
      .attr("cx", width/2)

svg.selectAll(".labels")
    	.data(randvals)
    	.enter()
    	.append("text")
	    .attr("class","labels")
    	.text((d)=>{return d})
      .attr("fill","black")
    	.style("text-anchor","middle")
    	.style("dominant-baseline","text-before-edge")
    	.style("font-size","11px")
      .attr("y", (d)=>{return  width - pad - radiusScale(Math.sqrt(d))*2})
      .attr("x", width/2)

</script>
</body>

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