text: HTML_TEMPLATE = """
  <!DOCTYPE html>
  <html>
  <head>
    <script src="https://cdn.jsdelivr.net/npm/vega@35></script>
    <script src="https://cdn.jsdelivr.net/npm/vega-lite@3"></script>
    <script src="https://cdn.jsdelivr.net/npm/vega-embed@3"></script>
  </head>
  <body>
    <div id="vis"></div>
    <script type="text/javascript">
      var spec = {spec};
      var opt = {opt};
      vegaEmbed("#vis", spec, opt);
    </script>
  </body>
  </html>
  """
  