on onclick="toggleHeatmap()">Toggle Heatmap</button>
      <button onclick="changeGradient()">Change gradient</button>
      <button onclick="changeRadius()">Change radius</button>
      <button onclick="changeOpacity()">Change opacity</button>
    </div>
-->
    <div id="map"></div>
    <script>

      // This example requires the Visualization library. Include the libraries=visualization
      // parameter when you first load the API. For example:
      // <script src="https://maps.googleapis.com/maps/api