41  Changes in State

Take-home Message

A change in visible state, needs to be appreciable and related to some actual change in the underlying variables.

Like all modes of communication, the visible mode relies of a change in state. That is, there must be some appreciable difference from before to after which conveys some communication. This is quite pertenant in the visible mode since it is, owing to its universality and ease of use, far too overused.

A change in state can occur in either a gradual rise or a step-wise change.

Visible Ramps
Changes in visible state on a continuous scale
Visible Steps
changes in visible state on a categorical scale

Imagine we standardize the amount of change, on any scale, that must occur before a change in state is appreciated as 1 unit. For any given use case, this begs the question, what is a 1 unit change related to?

A change in visible state needs to reflect some change in the actual variable. If it doesn’t, then we have a hard time to justify it beyond mere entertainment. If the visible change is difficult to perceive, then we must ask ourselves what goal does this serve?

41.1 Units of change

A variable’s units are know at the outset. For example we know the units for dose, weight, volumne distance, and so on. They may be transformed but they are know. The visual units we use to encode the variable’s units aren’t fixed, we choose them when we choose the visual mode of how we want to communicate. That is, what the actual size of 1 unit in visible change is defined by use during communication. By visible mode we can imagine the transparency of an object, its hue, lightness, or saturation (more on these in the color section), or for paths, their lengths, stroke thickness or end caps style. For polymorphic attributes we can consider the shape or an object. For writing we can imagine words on scales of intensity. Even in our body language — gestures, attire, facial expressions — exist on scales.

Let us take a moment to consider some changes in state that fall far below the 1 unit needed for an appreciable difference. For example, recognize that movies shot on film are on a physical media on a categorical scale. They are individual images which when presented with a very small unit, gives the illusion of a continuous motion. A movie then is a continuous transformation of a categorical variable. This is akin to continuous approximations of categorical theoretical probability distributions, where the change in state is so tiny that it vanishes and we imagine a continous spectrum.

Another example of imagining different units of change comes from a wonderful lyric from Laurie Anderson: You’re walking, and then you’re falling, and you catch yourself from falling. We don’t imagine walking as a form of constantly catching ourselves from falling, but if we slow down the rate of change Laurie Anderson hits the nail right on the head.

Extreme units of change also pose challenges. For example, if we expand the unit of change to geological or evolutionary time scales spanning millions of year, they become difficult to comprehend. We imagine stable worlds with low frequency change that have high impact changes. Mountains suddenly erupting overnight or fishes jumping out of water en mass and scurrying about. On the other end biological and chemical time scales are so fast that we see them as unstable, with a high frequency of change, with low impact changes compounding daily.

This is to say that how we perceive units of visible change determines how we understand the underlying process. We’ll keep this in mind as we review the many modes of visible communication in the following chapters.