Graphics Reference
In-Depth Information
Figure
.
.
Weekly Dow Jones Industrial Average:
a
Four years from
to
.
b
Six months from
July to December
. he maximum vertical axis value on the let is over four times the maximum on
the right
daily share prices only being available on days the market is open). Time units for
summary measurements may be of unequal length (e.g. months). he time period
chosen and the aspect ratio used for a time series plot can make a big difference in
the interpretation of the data (Fig.
.
).
If several time series are plotted in the same display, then it is necessary to en-
sure that they are properly aligned in time (e.g. two annual economic series may be
published at different times of the year), that their vertical scales are matched (the
common origin and the relative ranges) and that they can be distinguished from one
another. Depending on the data, this can be tricky to do successfully.
Maps
Geographic data are complex toanalyse, though graphical displays can be very infor-
mative. Bertin discussed many ways of displaying geographic data in his book, and
MacEachren's book contains a lot of sound advice (MacEachren,
),though more
from a cartographic point of view. he main problems to be solved lie in the fact that
areas donot reflecttherelative importance of regions (e.g.Montana hasfewerpeople
than NewYorkCitybutismuchbigger)andspatial distance isnotdirectly associated
with similarity or nearness (e.g. where countries are divided by natural borders, like
mountain ranges). here is a substantial research literature in geography on these
and other display issues, such as how to use colour scales to show values ('choropleth
maps')andhowtochoosecolourschemes(e.g.Colorbrewerreferredtoabove).Some
instructive examples can be found in the cancer atlas maps of US health authorities
on the Web and in the topic by Devesa et al. (
). Figure
.
shows that cancer
ratesarehighestalongtheEastCoastandlowestintheMidwest.StateEconomic
Areas (SEAs) have been chosen because using states oversmooths the data (consider
Nevadainthe Westwith its highcancer ratearound LasVegas,butits lowerrateelse-
where), while using counties undersmooths. he map on the website is in colour, on
a scale from deep red for high rates to dark blue for low. Naturally, this would not
reproduce well in a grey-scale view, so the webpage provides the alternative version
that is used here. Offering multiple versions of the same image on the Web is read-
ily possible but not oten done. his is one of several reasons why the cancer atlas
webpages are exemplary.
