The audience does not know where to look for to decode the graph objective. It is up to us to design a graph and guide the viewer to find the relevant information. Here enter the Gestalt principles of perceptual organisation, emanating from Gestalt psychology.
The Gestalt principles of perceptual organization describe how humans naturally perceive objects as organized patterns and objects rather than individual objects, irrespective of how they have been encoded.
To use the words of the pioneering Gestalt psychologist Kurt Koffka: “The whole is other than the sum of its parts.” The key idea is that the entire image, or in our case a data graph, is perceived independently from its encoding components.
Law of Prägnanz
The German word Prägnanz translates (about) to ‘precisely clear’.
The Gestalt theory is build on the idea of perceiving objects with Prägnanz, and the underlying premise that human cognition rationalises the surrounding environment in a collected, simplified, orderly and balanced manner.
This means that a data graph, a visual object, although defined by separate encoding components it is perceived as a whole as we seek to minimise visual stress in our physiological need for tranquility and resolved anxiety.
Thus, we perceive graphs as cohesive messages that we take at face value, and it takes a lot of cognitive effort to disentangle the individual encoding choices that give rise to the visual message. This is the idea of Emergence, now a well-accepted theory in the study of complex systems, which explains that an object has unique properties not contained in its parts.
For data graphs the Law of Prägnanz implies two important lessons.
First, the viewer first sees the whole image by itself and then tries to identify its components. If the whole requires a lot of cognitive effort to comprehend then the viewer may give up on trying to learn. So, the design of the overall image (the whole graph) is an important consideration, and should be produced as such as to attract attention and curiosity without the need to exercise excessive effort.
Second, once the viewer decides to learn more about the graph s/he will look immediately for a key message. The key message is conveyed wherever there is disorder, asymmetry or imbalance in the graph. This is because we seek to place an orderly meaning and if a piece of information is disordered, asymmetric and imbalanced then we will try to understand why this is so, and by exerting such cognitive effort will make us learn about the key message.
Given the Law of Prägnanz, Gestalt theory collects a number important principles that explain how we perceive the organisation of visual information:
- Figure/Ground: we perceive a contrast between foreground and background.
- Proximity: we perceive objects that are closer together as being related.
- Similarity: if proximity is not prevalent then we perceive similarities in retinal variables as definitions of grouping.
- Enclosure: we perceive objects with a boundary around them or enclosed within an area of common color as a related group.
- Closure: directly related to fictional illusions, we perceive imaginary shapes by connecting existing objects through empty space.
- Connection: we perceive objects connected by a line as a related group.
- Common Fate: we perceive objects that move in the same direction and at the same speed as related. This is an important consideration in the retinal variable of movement.
To learn more, follow the hyperlinks placed just above on each of the Gestalt principles.