The wood and the trees - The Visualization of Spatial Social Structure - page 234

8.4 Flocks of arrows

Overall, despite the decline in the class alignment among individuals,
social groups within the British electorate have not become more
politically homogeneous. Parliamentary constituencies have never
been more politically polarized and, in consequence, the number of
marginal constituencies held by small majorities has halved since
the 1960s.
(Miller, 1990, p. 49, referring to Curtice and Steed, 1988, p. 354)

Arrows can be used to show far more than geographical flow, as they do
(above) in Chapter 6. They are also a glyph that can satisfy many of the crite-
ria of visual simplicity. This is one of the simplest of signs, expressing mainly
orientation, although size and shape can also be incorporated to increase infor-
mation content. Its simplicity allows trajectories at many hundreds of places to
be simultaneously shown. Most importantly, at this level, the aggregate begins
to express a form of its own - the sum of its parts is greater than those parts as
a result of what imagination and intuition add to the overall image.
Like a flock of birds in flight, a group of arrows pointing in a similar direction
appears to be going that way; they become a visual group. Arrows can, for
example, indicate political flow. This is exactly the impression needed, and it is
through an analogy with a natural image that it becomes possible to do this.
Arrows have been used in many ways in this topic. The direction of the arrow
can represent the levels of two variables as a vector. Here they have been used
to show the three-party swing in constituencies between general elections. 7 The
arrow can point in the direction that a dot representing the constituency would
move on the electoral triangle. The length of the arrow can be used to show
another variable - the size of the swing. The size of the arrow is in proportion
to the electorate and its colour shows the proportions of the vote going to the
three major parties. The position of the arrow is dictated by the constituency
cartogram, which could be animated to show changes over time.
In one sense, nine dimensions were being seen in this relatively simple
picture - two for position, two for direction, three for colour and one for each of
length and size; but that would be a gross exaggeration. The position of the con-
stituency is shown by two dimensions, while the image is representing seven very
closely knit variables (only six are independent as direction is one-dimensional).
It is the strength of the relationships between the variables that allows so much
to be depicted. Ten elections' worth of results can be shown on an A4 page
containing over six thousand visible arrows (shown earlier).

7 The same electoral swing does not necessarily imply the same political behaviour in different
constituencies: 'In fact a uniform swing could only come about if a party's voters behaved differently,
not the same, according to the constituency in which they lived: a uniform 5 % swing from Labour
to Conservative logically requires Labour voters to defect at higher rates in hopeless seats than safe
seats. That this tended to happen reflected the “partisan neighbourhood” effect' (Crewe, 1988, p. 5).