Using d3-legend

This example demonstrates use of the d3-legend library.

The color scale comes from ColorBrewer.

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>D3 Legend Example</title>
    
    <script src="//cdnjs.cloudflare.com/ajax/libs/d3/3.5.6/d3.min.js"></script>
    <script src="//cdnjs.cloudflare.com/ajax/libs/d3-legend/1.1.0/d3-legend.js"></script>
    <script src="colorbrewer.min.js"></script>
  </head>
  <body>
    <svg width="960" height="500"></svg>
    <script>
      var svg = d3.select("svg");

// Color legend.
      var colorScale = d3.scale.quantize()
        .domain([ 0, 0.15 ])
        .range(colorbrewer.YlGn[9]);

var colorLegend = d3.legend.color()
        .labelFormat(d3.format(".2f"))
        .scale(colorScale)
        .shapePadding(10)
        .shapeWidth(30)
        .shapeHeight(30)
        .labelOffset(10);

svg.append("g")
        .attr("transform", "translate(200, 60)")
        .call(colorLegend);

// Size legend.
      var sizeScale = d3.scale.linear()
        .domain([0, 10])
        .range([2, 30]);

var sizeLegend = d3.legend.size()
        .scale(sizeScale)
        .shape("circle")
        .shapePadding(3)
        .cells(9)
        .labelOffset(10);

svg.append("g")
        .attr("transform", "translate(650, 60)")
        .call(sizeLegend);

</script>
  </body>
</html>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.6/d3.min.js

https://cdnjs.cloudflare.com/ajax/libs/d3-legend/1.1.0/d3-legend.js