code: function moveSubtree(wl, wr, shift) {
      var change = shift / (wr.hierNode.i - wl.hierNode.i);
      wr.hierNode.change -= change;
      wr.hierNode.shift += shift;
      wr.hierNode.modifier += shift;
      wr.hierNode.prelim += shift;
      wl.hierNode.change += change;
  }
docs: The implementation of this function was originally copied from "d3.js"
  <https://github.com/d3/d3-hierarchy/blob/4c1f038f2725d6eae2e49b61d01456400694bac4/src/tree.js>
  with some modifications made for this program.
  See the license statement at the head of this file.
  
  Shifts the current subtree rooted at wr.
  This is done by increasing prelim(w+) and modifier(w+) by shift.
  
  @param  {module:echarts/data/Tree~TreeNode} wl
  @param  {module:echarts/data/Tree~TreeNode} wr
  @param  {number} shift [description]