getLastAngle(vertexId) {
    while (vertexId) {
      let vertex = this.graph.vertices[vertexId];

      if (vertex.value.rings.length > 0) {
        // Angles from rings aren't useful to us...
        return 0;
      }

      if (vertex.angle) {
        return vertex.angle;
      }

      vertexId = vertex.parentVertexId;
    }

    return 0;
  }
Get the last non-null or 0 angle.
@param {Number} vertexId A vertex id.
@returns {Vertex} The last angle that was not 0 or null.