EDGE-EQUALIZED TREEMAPS - Information Visualisation: Techniques, Usability and Evaluation - page 122

distinguish leaf rectangles from the unused spaces produced by the layout
of leaf rectangles.

6.2.2 Branch rectangles

Fig. 6.10(a) shows a Treemap drawn by the above technique. Only leaf
nodes are filled with colours, and other nodes are represented as solid
colours. Moreover, branch nodes that include unused spaces seem to have
complicated shapes. The white spaces are eye-catching and seem to be
larger than they really are because white is bright. From these effects,
unused spaces tend to be seen as larger despite being reduced to the
minimum.
To resolve these problems, we give each branch node a colour (Fig.
6.10(b)). We fill each branch node with a hue, which is the average of its
child nodes' colours. However, unused spaces will appear to be nodes if
we fill them with the above colours. Thus, we use colours whose
brightness is higher than that of the child nodes for branch nodes.
The false recognition of unused spaces is reduced, and we can
distinguish the hierarchical level from the brightness of the nodes.
Furthermore, this prevents unused spaces appearing to be larger than they
are, because they are filled with colours that are darker than white.

(a) White unused spaces (b) Coloured unused spaces

Fig. 6.10. Colouring for branch rectangles

7 Use of the Edge-Equalized Treemap

We now demonstrate the use of our method by drawing a hierarchical
structure. Using an Edge-Equalized Treemap, we embed a bar chart into
each leaf rectangle, and equalize the scale intervals of the value axis
(vertical axis) in the Treemap.
Our Edge-Equalized Treemap shows rainfall data for different
prefectures. This data is organized in a hierarchy, with Honshu, Japan as