zoomOutViewLevel () {
    if (this.currentGraph) {
      if (this.currentGraph.highlightedObject) {
        this.currentGraph.highlightObject(undefined);
      } else if (this.currentGraph.graphIndex.length > 0) {
        this.setView(this.currentGraph.graphIndex.slice(0, -1));
      }
    }
  }
If zoomed into a node or a service, zoom out one level up.
If in the global view, this is a noop.