Tweemap treemap generator v1.0
Written by Luke Andrews <la@twitter.com>
Copyright (C) 2011 Twitter, Inc.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this
file except in compliance with the License. You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed
under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
CONDITIONS OF ANY KIND, either express or implied. See the License for the
specific language governing permissions and limitations under the License.

Uses algorithms described in:
  "Squarified Treemaps", by Mark Bruls, Kees Huizing, and Jarke J. van Wijk
  http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.36.6685&rep=rep1&type=pdf