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3 Ease of comparison between charts
We consider embedding charts with orthogonal coordinate axes into a
Treemap. Here, we suppose that charts are drawn on a two-dimensional
plane, and have two axes perpendicular to each other; two examples are
bar charts and area charts. If we wish to represent time-series data, many
charts will use the horizontal axis for time. When two or more charts have
the same time axis, the scale intervals of their axes should be equalized for
ease of comparison. Therefore, we aim to equalize the scale intervals of
the horizontal axes when we embed the charts into a Treemap.
3.1 Embedding charts into a Treemap
Fig. 6.3 shows a Squarified Treemap with embedded bar charts. This
Treemap represents the annual rainfall of Honshu,
1
Japan's main island.
Each rectangular area represents rainfall in one prefecture and the thick
borders represent individual regions. We can understand the annual rainfall
of a prefecture by observing the area of its corresponding rectangle. Also,
by studying the bar chart embedded in each rectangle, we can appreciate
monthly rainfall in a prefecture. The horizontal axis of the bar chart
expresses time and the vertical axis describes the amount of rainfall. Fig.
6.4 represents the same data using our proposed Edge-Equalized Treemap.
We discuss construction of this Treemap in detail later.
Problems with width variation:
As we see in Fig. 6.3, the leaf rectangles
have various widths. Moreover, different charts have different bar widths.
Therefore, if the scale interval of the vertical axis is equalized in a
Treemap, we cannot elicit quantitative values from the areas of the bars.
On the other hand, if we want to describe the same value using the same
area, it is not easy to equalize the scale intervals of the vertical axes.
Making a value in the chart correspond to both height and area is not
possible, and may cause misunderstandings.
1. There are 47 prefectures in Japan. These prefectures compose eight regions, five
of which are in Honshu.
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