rotateSubtree(vertexId, parentVertexId, angle, center) {
    let that = this;
    this.graph.traverseTree(vertexId, parentVertexId, function (vertex) {
      vertex.position.rotateAround(angle, center);

      for (var i = 0; i < vertex.value.anchoredRings.length; i++) {
        let ring = that.rings[vertex.value.anchoredRings[i]];

        if (ring) {
          ring.center.rotateAround(angle, center);
        }
      }
    });
  }
Rotate an entire subtree by an angle around a center.

@param {Number} vertexId A vertex id (the root of the sub-tree).
@param {Number} parentVertexId A vertex id in the previous direction of the subtree that is to rotate.
@param {Number} angle An angle in randians.
@param {Vector2} center The rotational center.