So far, no use has been made of the third piece of data in the coordinates list, which is the depth of the quake. This data will be presented in the form of a sorted list of the current features when the user presses the Sort button. Let's have a look at the following code snippet:

void sortFeatures(Event evt) {
  featPlotter.sortFeatures();
  DivElement out = querySelector('#depthDetail');
  out.nodes.clear();
  featPlotter.geoFeatures.forEach((feature) {
    LIElement detail = new LIElement();
    detail.innerHtml = "${feature[5]}km - ${feature[3]}";
    out.nodes.add(detail);
  });
}

The list of features is sorted in the method sortFeatures, and as Dart knows nothing of the meaning of the contents of the list, we provide a comparator function that decides which feature is deeper by comparing the depth measurement held at index 5 of the list:

  List sortFeatures(){
    geoFeatures.sort( (a,b) => a[5] - b[5] );
    return geoFeatures;
  }

The comparator receives...