Shneiderman, B. (March 1991)
The traditional approach to representing tree structures is as a rooted, directed graph with the root node at the top of the page and children nodes below the parent node with lines connecting them has a long discussion about this standard representation, especially why the root is at the top and he offers several alternatives including brief mention of a space-fillling approach . However, the remainder of his presentation and most other discussions of trees focus on various node and edge representiation. By contrast, this paper deals with a two-dimensional (2-) space-filling approach in which each node is a rectangle whose area is proportional to some attribute such as node size.