Data visualisations should represent their underlying data as accurate as possible, and timelines are no exception. However in many cases, temporal data is not accurate in the first place, as it can not easily be measured or counted. In order to represent such uncertain data accurately, we have to allow for ambiguousness in the visual representation of it.
A visualisation should make understandable through an image, what is difficult to grasp in words, and enable new discoveries through visual analysis. Data visualisation in particular enables abstract numbers to be compared visually, or patterns to emerge from what might be just a list of measurements. It is always a translation, a representation of information in a graphical format, which by itself does not contain any new information in the strict sense 1 that was not contained in the raw data already. In the first instance, it makes existing information accessible for visual exploration.
Joseph Priestly stresses this aspect of his work in his Description of a Chart of Biography. The text accompanies what can be considered one of the first graphical timelines (after the pioneering work by Jacques Barbeau-Dubourg): A chart depicting the lives of about two thousand individuals, represented by lines on a linear scale of years ranging from 1200 to 1800.
It is of course an understatement when Priestley declares himself simply “to be an assistant to the great Historians, Chronologers, and Biographers” (p.4), whose work forms the foundation of his timeline. What he means, is that he did not himself produce any new knowledge, but assembled the research of others in one coherent visualisation — laborious and painstakingly of course. In reward for his undertaking, he was now able to see and also show to others, the relation and succession of historic figures, their contemporaries with whom they might have conversed, and the periods when cultural life flourished or, symbolised by emptiness, stalled. The representation of chronological information in graphical form opened it up for visual analysis and exploration, which enables new hypotheses to be developed from which ultimately new knowledge can be gained.
I mention this historic example of a graphical timeline, because it exhibits awareness for key requirements a timeline needs to fulfil, which are still relevant today. Being one of the first to ever create a timeline, Priestley gave careful consideration to all of his design decisions, which he documented in his Description.
Accuracy in inaccuracy
His main concern and a requirement which every data visualisation should respect, is accuracy. If one uses a visualisation as a basis for research, even slight inaccuracies can easily and significantly affect the research outcome. Priestley’s forerunner, Barbeu-Dubourg who created a similar timeline of biographies shortly before and independent of Priestley’s work, used a graphical representation which lacked a certain level of accuracy. He did use a linear mapping from time to diagrammatic space, something which no one is known to have done before him. He then drew icons along with written names to position the individual lives along the timeline. One could therefore only guess the precise dates of birth and death of every individual. While it was possible to gain an impression of the time certain persons lived, the representation did not allow for a more precise analysis.
Priestley’s innovation was to draw a line, ranging from birth to death, for every individual on the timeline. “A line”, he argues, “which, like time, may be extended in length, without giving any idea of breadth or thickness” (p. 6) seemed for him to be the most legible representation of time, and time has certainly proven him right. The lines allow the viewer of Priestley’s chart to read from the graphics alone the lifespan of an individual with precision of up to a year.
The vicious thing about time however, is that it is not as absolute and precise as it is often treated. It’s easy to miss a Skype meeting, if you agree with your correspondent on a time, but each one has his own time zone in mind. When working with historic dates, the inaccuracies tend to increase. Even when a date is precisely recorded, it is not necessarily measured in line with the calendar we use today. Often, several dates are associated with a single event, depending on which source one consults and in many cases a date is only roughly known, even when it is treated otherwise 2.
Priestley did acknowledge these problems and tackled them in different ways. When authors disagree on a specific date, he put his confidence in “those who give reasons for the dates they assign, or those who seem to have considered the subject with the most attention.” (p. 13). While his choice was not arbitrary, he did select a single account to represent, omitting all the others. The user of his chart is left unaware of some of the conflicting information behind the depicted dates.
In terms of the confidence and accuracy of the dates, he did introduce a visual notation system to indicate uncertainty by using dots and broken lines. If the death of an individual is known, but not his birth, the line would begin as dots and end in full. Similar graphical notations are used to express different levels of uncertainties. Unlike the curatorial decisions of the designer, the quality of the underlying data is reflected in the timeline visualisation.
Why uncertainty is neglected
Today, we might know events for certain that were wild guesses in Priestley’s time, and through technologies such as carbon dating it is possible to determine the age of organic material with high accuracy (in relation to the age of the universe). Possibly, such advances in sciences and in measurement of time are one reason, why contemporary timelines tend to assume a certain accuracy and definition, which is not necessarily present in the underlying data.
Another reason can be found in the manner that dates and times are recorded, namely in databases. While modern collection management systems support entering an implicit estimation in natural language for, say, the production date of an object, these details are still translated internally into an explicit date format. When a user enters “in the 18th century”, the database may store the first of January 1700 at 00:00 as the earliest possible date and the 31st of December 1799 at 23:59 as the latest date. Different interpretation algorithms may generate different dates, but in essence, the nature of a computing system expects exact values. The term “in the 18th century” becomes a clear-cut timespan when it actually signifies a relatively vague indication.
In order to design timelines which do not pretend to present facts with greater precision than their underlying evidence would allow, we need to keep the following in mind:
1. We have to be careful to not put too much confidence in data, especially when the data refers to something in the past and/or there is not enough evidence for the data.
2. Data often has been adapted or interpreted in order to fit into a certain structure. It might be necessary to break free from it.
3. Tools, not only digital ones, can enhance but at the same time restrict our thinking. If all you have is a hammer, everything looks like a nail.
At a later stage, I will present how we can tackle these problems on the level of both digital data structures and visual representation.
|||J Priestley, A Description of a Chart of Biography, 1764|
|||S Boyd Davis, Joseph Priestley: The Man Who Drew Time, 2011|