Visualisation Techniques for Temporal Information
These pages are a high level overview of graphical representations of temporal information. They are intended to be a short personal guide to the relevant literature and applications. The intention is to encourage further reading and research, rather than being a definitive review.
Introduction and Context
I’ve trawled through several historical reviews [1], looking for timeline innovations. Most early representations of time were cyclical [2]. Here is a great relatively recent example by Florence Nightingale.
Within the of historic evolution of timelines [3] the first linear axis timelines appeared around the mid 18th century.
The vertical dimension of the main data area is almost always used to group related types of information. Horizontal lines denote duration, or related events. Vertical bands help the eye line up and compare information.
It is remarkable that this basic format is still the most commonly used approach to representing temporal information today.
Non-linear versions appeared much later. Another variant is to orient flow time top to bottom on the page, although left-to-right orientations are much more common.
Other notable representations include calendars of course.
Simple graphs (histograms, pie charts) of statistical counts are often used to show overall patterns.
Spiral representations are often rediscovered.
Spirals are appropriate for some tasks because they have cycles (e.g., months above) as well has a linear progression (generally outwards from the centre). Here is a rectangular variant from Wijk & Selow (pdf) that has more regularity in terms of space available per time slot.
A Graphics Context
Of the seven ‘visual variables’ of Jacques Bertin below, most time visualisations use some sort of position to represent time.
The basis for these choices may be MacKinlay’s diagrams (pdf) below concerning the suitability of visual variables for data types. A general picture:
And now applied to certain data types:
Given this model then, it isn’t surprising that most approaches to time visualisations use position to represent time. Most commonly, colour, value (dark/light), form and size are used to represent the temporal data. Colour and value are frequently used for visualising cyclical patterns (weekends, months, hours of darkness, etc.). Form tends to be used for qualities which do not change over time, such as categories.
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