Chapter 17. The Principle of Proportional Ink
In many different visualization scenarios, we represent data values by the extent of a graphical element. For example, in a bar plot, we draw bars that begin at 0 and end at the data value they represent. In this case, the data value is not only encoded in the endpoint of the bar but also in the height or length of the bar. If we drew a bar that started at a different value than 0, then the length of the bar and the bar endpoint would convey contradicting information. Such figures are internally inconsistent, because they show two different values with the same graphical element. Contrast this to a scenario where we visualize the data value with a dot. In this case, the value is only encoded in the location of the dot, not in the size or shape of the dot.
Similar issues will arise whenever we use graphical elements such as bars, rectangles, shaded areas of arbitrary shape, or any other elements that have a defined visual extent which can be either consistent or inconsistent with the data value shown. In all these cases, we need to make sure that there is no inconsistency. This concept has been termed as the principle of proportional ink [Bergstrom and West 2016]:
When a shaded region is used to represent a numerical value, the area of that shaded region should be directly proportional to the corresponding value.
(It is common practice to use the word “ink” to refer to any part of a visualization that deviates from the background color. ...