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The spike shape acts as a pointer in the direction of the set labels
around the perimeter of the Venn diagram. A viewer can instantly
understand how many segments that the circle corresponds to (i.e. the
number of spikes around the perimeter of the circle) and the specific sets
that the circle corresponds to (i.e. by referring to the direction that the
spike points and mapping that to the corresponding label around the
perimeter). Perceptually, it is similar to the task of reading the face of an
analogue clock. This use of shape as a pointer is potentially easier to
interpret than either decoding the colour corresponding to the particular
segment back to each corresponding set colour; or visually tracing all the
underlying Venn diagram outlines back to each respective source. Further,
the spike approach is extensible to higher-order Euler diagrams [Bra12].
Fig. 3.30.
A Venn diagram of Titanic survivor data, with a bubble per segment
sized to indicate the number of corresponding passengers; and with spikes per
bubble to indicate set membership by pointing towards the corresponding set labels
around the perimeter. For example, the large bubble near the centre bottom has
three spikes, indicating that its members belong to three sets. The orientation of
these spikes correspond to the location of the labels around the perimeter;
therefore, based on the spikes it can be determined, this large bubble corresponds
to a large number of 1
st
/2
nd
class, female passengers that survived the Titanic
disaster.
Shape as a frame of reference
Rather than using shape attributes to form a glyph, shape attributes can be
used to form a common reference upon which data is displayed, much like
Tufte's Small Multiples [Tuf90]: the common reference in each frame
provides a basis to locate and compare visual elements across each frame.
In this case, the reference shape could represent data structures such as
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