9.3 Tree-Maps: Visualization of Hierarchical Information Structure

Tree-Maps represent a novel way for the space-efficient representation of hierarchical information [Joh91]. Johnson and Shneiderman's work has been motivated by the problem of visualizing large directory structures on hard disk drives. Tree-Maps therefore are specially well suited for the representation of very large tree-hierarchies. A Tree-Map drawing encompasses the following properties [Joh91]:

• If node1 is an ancestor of node2, then the bounding box of node1 completely encloses, or is equal to, the bounding box of node2.
• The bounding boxes of two nodes intersect only if one node is the ancestor of the other.
• Nodes occupy a display area strictly proportional to their weight.
• The weight of a node is greater than or equal to the sum of the weights of its children.

This means in particular that each node has an associated weight. This weight defines the area that the node occupies in the Tree-Map. Figure I.38 displays a tree in the conventional graph tree representation and as a Tree-Map. As figure I.38 illustrates, only nodes that are leaves are shown directly. They indicate common ancestry by using similar patterns or colors. A slice and dice algorithm, that rotates the orientation of the elements by 90^o on each hierarchy level, produces an appealing layout of the elements.

Figure I.38 Graph tree and Tree-Map of the same graph

Tree-Maps are especially useful for giving a quick overview if the hierarchy is very large and contains thousands of nodes. In particular, they allow the easy transmission of an additional dimension to the user by varying the node size depending on the weight. By using meaningful color coding, even a third dimension can be added to the Tree-Map display.

Tree-Maps offer an unorthodox approach to the information exploration problem for hierarchically structured data. By compressing the whole structure on a computer screen, the user gets direct navigational cues and the problem of being lost in hyperspace is therefore being tackled from another side. The addition of zooming features to a one-level Tree-Map allows the user to go into arbitrary levels of detail. Tree-Maps thus offer an additional fourth way of representing hierarchical structure extending Knuth's listing of methods for displaying trees.