What has caught my attention when reading Priestley’s Description Of A Chart Of Biography, is how well one can relate to his process and the decisions he took when working on the chart, although he was designing for an entirely different audience and medium than we do today. It seems that the process for designing information visualisations has not changed significantly in more than 200 years.
I have extracted from his descriptions four general rules, that Priestley seemed to adhere to (consciously or not) and that I and probably many others still apply today:
- Establish Guidelines
- Before one can begin to put data into visuals, it is important to define some guidelines on how this transformation will occur. For it is always a transformation and only when it is performed in the same way for every instance of data, it is possible to contextualise and compare.
- Allow Interpretation
- There will be cases where a transformation can not be performed in the way intended. At first, one should check whether this might be due to a mistake in the guidelines and, if yes, adapt the guidelines for all elements. However, often this can lead to things feel right in one case, and feel awkward in all the others, at which point it is important to trust one’s instinct as a designer and break one’s own rules.
- Allow Redundancy
- Tufte’s Data-Ink Ratio is an encouragement to remove clutter from data visualisation, however one should not refrain from displaying redundant information when it helps the usability and understandability of a graphic.
- Allow Involvement
- A visualisation should not be designed to simply be looked at, but to be used. During the design process one should keep in mind how a user might potentially interact and interfere with the graphic, be it a printed, digital or physical implementation.
After the jump, I will show by example how those rules have been applied by Priestley.
While Priestley may not be the first person ever to create a timeline — he was at least preceded by Jacques Barbeu Du Bourg — Priestley did in fact introduce a design concept which was novel: the representation of time as a line. This translation seemed as natural to him then as it feels to us now:
THUS the abstract idea of TIME, though it be not the object of any of our senses, and no image can properly be made of it, yet because it has a relation to quantity, and we can say a greater or less space of time, it admits of a natural and easy representation in our minds by the idea of a measurable space, and particularly that of a LINE ; which, like time, may be extended in length, without giving any idea of breadth or thickness
Priestley 1778, p. 6
Very much aware of the fact, that data is always tied to a level of confidence, Priestley looked for a way to express uncertainty in the data presented.
[…] especially the time of the birth of eminent men cannot always be found. Priestley 1778, p. 11f
A full line expresses certainty while dots or a broken line represent vagueness at different levels. One single dot stands for a fair amount of confidence, while three dots represent an uncertain birth or death. Often, little was known about the exact dates when a person lived, apart from the fact that “a writer flourished at or about a particular time”. Priestley established guidelines for how to represent such vagueness, positioning two thirds of a line before the specified time and letting it start and end in three and two dots respectively.
[…] a short full line is drawn about two thirds before and one third after that particular time, with three dots before and two after it; because, in general, men are said to flourish much nearer the time of their death than the time of their birth
Priestley 1778, p. 11
Priestley used the horizontal axis of his chart to represent time, in equal spacings and divided into centuries. Naturally, the data also needed to be separated vertically in order not to overlap. Although this could be done at random, Priestley recognised the benefit of providing more information and structure by making use of this additional dimension in a sensible way. The vertical axis is divided into categories, six in total: Statesman and Warriors, Divines and Metaphysicians, Mathematicians and Physicians, Poets and Artists, Orators and Critics and Historians and Antiquarians.
Every name falls within one and only one of those categories, although some possibly could be placed in both. If that’s the case, Priestly made sometimes use of the vertical positioning within a certain category and positioned ambiguous cases accordingly:
[…] the Popes, as they partake of the nature both of Divines and Statesmen, I have placed next to the Statesmen among the Divines
Priestley 1778, p. 20
Last, but not least, Priestley describes his selection criteria for the names represented and, maybe more importantly, not represented in the chart. In a way, his approach is very similar to Google’s PageRank algorithm, that judges the importance of a web page by the amount and rank of links pointing to it. Not the quality of the content is relevant, but the popularity of it — “ […] renown not merit” (p. 16) in Priestley’s words.
so that a person who had made a great noise in the world, though he were known by nothing but the devastation he had made in it, was more acceptable to me than one who had deserved ever so well of it
Priestley 1778, p.16
Priestley makes the case, that his graphical representation is in many ways superior to textual catalogue. Nevertheless, he decided to include the same information that is plotted visually also in a tabular format.
[…] several reasons have induced me […] to draw up a catalogue of all the names inserted in it, and to annex the real dates to each of them.
Priestley 1778, p.12
The bespoken reasons where the fact that, although the lines were drawn with uttermost precision, one could only with great difficulty read off the exact dates of birth and death. Also, it is very hard to find a name within the chart, if one does not have a general idea in which century or category to look for.
In a sense every fact in the chart is represented twice, in graphics and writing, and therefore partly redundant. The gain in usability of the chart however more than justifies the redundancy in information.
Perhaps most insightful in Priestley’s Description are not the sections where he outlines his visual and syntactical guidelines. After all, most decisions can be derived from the chart itself. Where one can really gain an impression of his design process is when he gives reason for breaking his own conventions, which he did on various occasions.
For example, in “busier” centuries, where a lot of names would deserve a place in the chart, the selection criteria employed were tougher and a lot of them were excluded due to a lack of space.
[…] hundreds which have been excluded in later, and more crowded ages, would have found a ready reception in an earlier period
Priestley 1778, p. 17
Where excluding a name was not an option, Priestley moved it to the second best fitting category:
[…] several persons finding no room among the Divines, were obliged to content themselves with a place among the Historians or Critics.
Priestley 1778, p.21
Although he followed some guidelines in the vertical positioning of names within a category, often it was his interpretation and sometimes even his subjective amusement that was decisive:
[…] placing those persons the nearest together who had the most connections, and whom I thought it would be most amusing to compare together
Priestley 1778, p. 19
In fact Priestley does not deny the fact, that personal preferences and involvement rather than objectivity have often influenced the design of the chart (pp. 13, 17, 18).
To make up for the fact that a user might not agree with the selection of names in Priestley’s chart, he asserts that there is enough room to also add own lines at will. Except maybe in the most crowded places, for which he offers a workaround:
[…]Besides, lines may be drawn in any place, even where names cannot be inserted; and as the person who inserts them will know what lives they represent, the names are quite superfluous.
Priestley 1778, p.18
He also leaves space for the user to update the chart himself, at least until the unforeseeable future of 1800, “a date which none who are now capable of perusing this chart can reasonably expect to see” (p. 18).