treemap.paddingBottom()

treemap.paddingBottom([padding])

If padding is specified, sets the bottom padding to the specified number or function and returns this treemap layout. If padding is not specified, returns the current bottom padding function, which defaults to the constant zero. If padding is a function, it is invoked for each node with children, being passed the current node. The bottom padding is used to separate the bottom edge of a node from its children.