Create legend for proportional circles in OpenLayers

<!DOCTYPE html>
<html>
  <head>
    <meta charset=utf-8>
    <meta name=description content="">
    <meta name=viewport content="width=device-width, initial-scale=1">
    <title>Thematic map with legend</title>
    <link rel="stylesheet" href="https://openlayers.org/en/v4.6.4/css/ol.css" type="text/css">
    <style>
      html, body {
        height: 100%;
        padding: 0;
        margin: 0;
      }
      #map {
        width: 100%;
        height: 100%;
      }
      #popup {
        background-color: white;
        border: solid 1px #888888;
        padding: 10px;
        border-radius: 5px;
      }
      #container {
        position: relative;
        height: inherit;
        width: inherit;
      }
      #canvas, #map {
        position: absolute;
        background-color: white;
      }
      #canvas {
        bottom: 0px;
      }
    </style>
    
    <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=requestanimationframe,element.prototype.classlist,url"></script>
    <script src="https://openlayers.org/en/v4.6.4/build/ol.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/papaparse/4.3.6/papaparse.min.js"></script>
  </head>
  <body>
    <div id="container">
      <div id="map" class="map"></div>
      <div id="popup"></div>
      <canvas id="canvas"></canvas>
    </div>
  <script src="main.js"></script>
  </body>
</html>

Updated missing url https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL to https://cdn.polyfill.io/v2/polyfill.min.js?features=requestanimationframe,element.prototype.classlist,url

Updated missing url https://cdnjs.cloudflare.com/ajax/libs/PapaParse/4.3.6/papaparse.min.js to https://cdnjs.cloudflare.com/ajax/libs/papaparse/4.3.6/papaparse.min.js

https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL

https://openlayers.org/en/v4.6.4/build/ol.js

https://cdnjs.cloudflare.com/ajax/libs/PapaParse/4.3.6/papaparse.min.js