Database Reference
In-Depth Information
C H A P T E R 11
Timelines
Providing an understanding how values change over time is one of the most
fundamental functions of visualization, and the presentation of this type of
information has evolved significantly since it was invented. Timeline visual-
izations serve as an aid to comprehension, allowing you to extrapolate and
predict a trend. For example, if sales are consistently increasing by $100,000
per month over the last three months, it is easy for the human eye to imagine
an increase over the upcoming months. Similarly, seasonality, such as the sales
increases in the retail sector for the Christmas shopping season, becomes easy
to see as a chart line rises and falls. Trends can be built into the charts as well
as trendlines. Care must be taken when using trendlines to ensure that the
user of the chart is kept aware of the fact that trends are derivations based
on the data rather than actual measured values. Statistical predictions are
another way of creating trendlines.
hIsTorICAl use oF TemPorAl AnAlysIs
The earliest examples of timeline visualizations were in the mid-17th century,
when Joseph Priestly created a timeline to compare the lifespan of various
people, as illustrated in the “Timelines” section later in this chapter, and the
convention of time being shown as running left to right, used to match
the direction in which the names on the timeline have been read is one
that we continue to use for visualizations. William Playfair, who you read
about in Chapter 2, extended this usage by creating bar, column, and line
charts to show changes over time. Although both bar and column charts
are often used purely for comparison, their original use was to allow com-
parisons over time, or to show a progression over time.
