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the root node; the child nodes show regions, and their child nodes show
prefectures (these are leaf nodes). Leaf nodes show the values of monthly
rainfall, and use annual rainfall as their weights.
Fig. 6.4 shows the result of this execution. The area of each rectangle
represents the annual rainfall of each prefecture. A bar chart represents
monthly rainfall, with the horizontal axis denoting the month and the
vertical axis showing the amount of rainfall.
8 Discussion
We compared our Edge-Equalized Treemap to an existing representation.
We represented the annual rainfall data of Honshu, Japan, using a
Squarified Treemap, as discussed in Section 3.1. We represented the same
data using the Edge-Equalized Treemap described in Section 7. We
compared and evaluated the two representations.
8.1 Various widths
When using a Squarified Treemap, the leaf rectangles have various widths,
such that the widths of the bar charts cannot be equalized. Because of this,
the values in the bar chart cannot correspond to both the height and the
area of the bar. In other words, we cannot accurately compare the charts
just by looking at them.
In the Edge-Equalized Treemap, the widths of all leaf rectangles are
equalized and the widths of all bar charts are also equalized. Therefore, the
values in a bar chart can correspond to both the height and the area of the
bar, and the interval scale of the horizontal axes can be equalized. Thus,
we can compare charts by simply looking at them.
8.2 Margins at the top of rectangles
In the Squarified Treemap, the height of the leaf rectangles does not
correspond to the weight of the leaf nodes. Therefore, there are some large
margins at the top of the charts. In other words, the drawing area is not
efficiently used.
The Edge-Equalized Treemap represents the weight of a leaf node as the
height of a leaf rectangle. The weight is the sum of the data drawn in the
chart. Thus, the drawing area of the chart has a height corresponding to the
value of the data. Because of this, there are no large margins at the top of
the leaf rectangles. In other words, the height of the leaf rectangle is
appropriate.
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