Reset dendrogram height attributes.Source:
reposition is a helper function used for manually creating
"dendrogram" objects from nested lists. The function
recursively reassigns the 'height' attributes at each node by
a given constant.
an object of class
either the character string "reset" (shift the graph so that the height of the farthest leaf from the root is zero), or a numeric value giving the amount to shift the graph along the primary axis.