code: function injectEdgeLabelProxies(g) {
    _.forEach(g.edges(), function(e) {
      var edge = g.edge(e);
      if (edge.width && edge.height) {
        var v = g.node(e.v);
        var w = g.node(e.w);
        var label = { rank: (w.rank - v.rank) / 2 + v.rank, e: e };
        util.addDummyNode(g, "edge-proxy", label, "_ep");
      }
    });
  }
docs: /*
  Creates temporary dummy nodes that capture the rank in which each edge's
  label is going to, if it has one of non-zero width and height. We do this
  so that we can safely remove empty ranks while preserving balance for the
  label's position.