Circles Legend

When you encode data with a circle, create a legend using the same scale.

<!DOCTYPE html>
<html>
<head>
	<style>
		body {
			margin: 0;
			font-family: "Helvetica", sans-serif;
		}
		#legend {
			margin: 0 auto;
			display: table;
		}
		.legend-circle {
			fill: none;
			stroke: #000;
		}
		.legend-dotted-line {
		  stroke: #000;
		  stroke-dasharray: 2, 2;
		  shape-rendering: crispEdges;
		}
		.legend-number {
		  font-size: .7em;
		}
	</style>
</head>
<body>
	<div id="legend"></div>
	<script src="https://d3js.org/d3.v4.min.js"></script>
	<script>
		 var data = [{name: "Andrew", value: 237}, {name: "Bob", value: 120}],
		 	variable = "value",
		 	max_circle_size = 50,
		 	max_data = d3.max(data, d => d[variable]),
		 	circle_scale = d3.scaleLinear()
			    .range([0, max_circle_size])
			    .domain([0, max_data]),
			legend_text_left_pad = 8,
			legend_width = max_circle_size * 3,
			legend_height = max_circle_size * 2 + 10,
			legend = d3.select("#legend").append("svg")
					.attr("width", legend_width)
					.attr("height", legend_height),
			legend_data = [max_data, 150, 50],
			legend_circle = legend.selectAll(".legend-circle")
						.data(legend_data)
			    .enter().append("circle")
			      .attr("class", "legend-circle")
			      .attr("cy", d => circle_scale(d) + 1)
			      .attr("cx", circle_scale(max_data) + 1)
			      .attr("r", d => circle_scale(d)),
			legend_dotted_line = legend.selectAll(".legend-dotted-line")
					.data(legend_data)
		    .enter().append("line")
		      .attr("class", "legend-dotted-line")
		      .attr("x1", circle_scale(max_data) + 1)
		      .attr("x2", circle_scale(max_data) * 2 + legend_text_left_pad)
		      .attr("y1", d => circle_scale(d) * 2 + 1)
		      .attr("y2", d => circle_scale(d) * 2 + 1),
		  legend_number = legend.selectAll(".legend-number")
		      .data(legend_data)
		    .enter().append("text")
		      .attr("class", "legend-number")
		      .attr("x", circle_scale(max_data) * 2 + legend_text_left_pad)
		      .attr("y", d => circle_scale(d) * 2 + 5)
		      .text((d, i) => d + (i == legend_data.length - 1 ? " " + variable : ""));
	</script>
</body>
</html>

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