1.1 What are good graphics?

Whether someone thinks a graphic is good or not is usually a matter of taste. It is easier to agree on graphics that are not good, even if people may disagree about why they are not good. Good graphics convey information clearly in a structured way.

Sometimes it is fairly obvious that a graphic could be improved: the display might be too small or too large, too thin or too broad; the labelling might be inadequate, confusing or overdone; the colours might be garish or indistinct; lines might be too thin or too thick; another type of graphic might display the information more effectively. The intention behind the graphic may be sensible, but the implementation may be weak.

Misleading graphics are a much more serious problem, and these are the ones that are really not good. They distract from information in the data or hide information, inhibit relevant comparisons, direct attention to unimportant features. So it is more a matter of how and why than good and bad. You can see why a good graphic has been drawn because of the way it has been drawn.

Several criteria have been suggested for assessing the quality of graphics including accuracy in reading off numbers, clarity of message, speed in recognising information, engagement, memorability. None are ideal and most are not relevant to exploratory graphics. A good exploratory graphic is one from which much information can be derived relatively easily, and in which features and patterns can be readily recognised and informatively interpreted in the context of the dataset. Many studies have been carried out evaluating how well viewers can assess numerical values from graphics. Graphics are not good for this; they are better for revealing and displaying information. It is the conclusions that can be drawn, not the numbers that matter. Exploratory graphics should be evaluated as to what information they provide and how well they do it. Much depends on the knowledge and experience of the analysts.

Assessing graphics, particularly complicated ones or collections of them, is not easy. Hoare’s remark on software design could be adapted for graphics: “I conclude that there are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is make it so complicated that there are no obvious deficiencies.” (Hoare (1981))