In watches, a complication is anything that goes beyond the basic function of showing the current time: alarm time, moon phase, etc. I think the term should be adopted in user interface design and visualization.
With their upcoming Watch, Apple is clearly playing to horology and the long history behind the design of classic watches. They use many watch terms, even where they don’t really make sense – like movement for the watch core CPU (which has no moving parts and does way more than a typical movement that just moves the hands).
Among the Watch’s complications are tiny widgets that show the current temperature, a stock price, the wearer’s activity level, etc. They can be turned on and selected by the user.
What does this have to do with visualization or user interfaces? I think there is a deep cleverness in calling these things complications beyond just the cutesy superficial one.
Anything that does not serve the basic function of a watch, user interface, or visualization should be called a complication. That doesn’t mean it needs to be removed. After all, complications can make a watch unique and useful. But it needs to be questioned. It needs to be examined and weighed against the distraction and clutter it causes.
In a watch, it’s clear what its central function is. That is often not nearly as clear in an application or a visualization. But thinking about it in terms of complications might help: if I remove this element/button/label/data series, does it still work? If it does, do I still want to keep the element?
This is similar to the idea that design is about reducing things to their minimal functional and aesthetic core. Thinking of design elements as complications rather than in strict terms like the data-ink ratio turns the question from a supposed rule to one of figuring out the trade-offs. And those can be quite different depending on the goals and the context. A useless item in one might be crucial to understanding a piece or engage an audience in another.
Complications as a concept is nice because it opens up a conversation. It moves us beyond supposedly strict and straightforward rules that seem to be set in stone. Many things are more complicated than that.
Teaser image from Wikimedia, used under creative commons.