treemap.paddingTop()

treemap.paddingTop([padding])

If padding is specified, sets the top padding to the specified number or function and returns this treemap layout. If padding is not specified, returns the current top 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 top padding is used to separate the top edge of a node from its children.