handlePolygonClick(event) {
    this.clear();
    var feature = event.feature;

    // Instantly highlight the polygon and show the title of the polygon.
    this.map.data.overrideStyle(feature, {strokeWeight: 8});
    var title = feature.getProperty('title');
    $('.panel').show();
    $('.panel .title').show().text(title);

    // Asynchronously load and show details about the polygon.
    var id = feature.getProperty('id');
    $.get('/details?polygon_id=' + id)
        .done((function(data) {
                if (data['error']) {
                  $('.panel .error').show().html(data['error']);
                } else {
                  $('.panel .wiki-url').show().attr('href', data['wikiUrl']);
                  this.showChart(data['timeSeries']);
                }
              }).bind(this));
  }
Handles a on click a polygon. Highlights the polygon and shows details
about it in a panel.
@param {Object} event The event object, which contains details about the
    polygon clicked.