25 Avoid line drawings
Whenever possible, visualize your data with solid, colored shapes rather than with lines that outline those shapes. Solid shapes are more easily perceived, are less likely to create visual artifacts or optical illusions, and do more immediately convey amounts than do outlines. In my experience, visualizations using solid shapes are both clearer and more pleasant to look at than equivalent versions that use line drawings. Thus, I avoid line drawings as much as possible. However, I want to emphasize that this recommendation does not supersede the principle of proportional ink (Chapter 17).
Line drawings have a long history in the field of data visualization because throughout most of the 20th century, scientific visualizations were drawn by hand and had to be reproducible in black-and-white. This precluded the use of areas filled with solid colors, including solid gray-scale fills. Instead, filled areas were sometimes simulated by applying hatch, cross-hatch, or stipple patterns. Early plotting software imitated the hand-drawn simulations and similarly made extensive use of line drawings, dashed or dotted line patterns, and hatching. While modern visualization tools and modern reproduction and publishing platforms have none of the earlier limitations, many plotting applications still default to outlines and empty shapes rather than filled areas. To raise your awareness of this issue, here I’ll show you several examples of the same figures drawn with both lines and filled shapes.
The most common and at the same time most inappropriate use of line drawings is seen in histograms and bar plots. The problem with bars drawn as outlines is that it is not immediately apparent which side of any given line is inside a bar and which side is outside. As a consequence, in particular when there are gaps between bars, we end up with a confusing visual pattern that detracts from the main message of the figure (Figure 25.1). Filling the bars with a light color, or with gray if color reproduction is not possible, avoids this problem (Figure 25.2).
Next, let’s take a look at an old-school density plot. I’m showing density estimates for the sepal-length distributions of three species of iris, drawn entirely in black-and-white as a line drawing (Figure 25.3). The distributions are shown just by their outlines, and because the figure is in black-and-white, we’re using different line styles to distinguish them. This figure has two main problems. First, the dashed line styles do not provide a clear separation between the area under the curve and the area above it. While our visual system is quite good at connecting the individual line elements into a continuous line, the dashed lines nevertheless look porous and do not serve as a strong boundary of the enclosed area. Second, because the lines intersect and the areas they enclose are not shaded, we can perceive six different distinct shapes and we need to expend some additional mental energy to merge the correct shapes together. This effect would have been even stronger had I used solid rather than dashed lines for all three distributions.
We can attempt to address the problem of porous boundaries by using colored lines rather than dashed lines (Figure 25.4). However, the density areas in the resulting plot still have little visual presence. Overall, I find the version with filled areas (Figure 25.5) the most clear and intuitive. It is important, however, to make the filled areas partially transparent, so that the complete distribution for each species is visible.
Line drawings also arise in the context of scatter plots, when different point types are drawn as open circles, triangles, crosses, etc. As an example, consider Figure 25.6. The figure contains a lot of visual noise, and the different point types do not strongly separate from each other. Drawing the same figure with solidly colored shapes addresses this issue (Figure 25.7).
I strongly prefer solid points over open points, because the solid points have much more visual presence. The argument that I sometimes hear in favor of open points is that they help with overplotting, since the empty areas in the middle of each point allow us to see other points that may be lying underneath. In my opinion, the benefit from being able to see overplotted points does not, in general, outweigh the detriment from the added visual noise of open symbols. There are other approaches for dealing with overplotting, see Chapter 18 for some suggestions.
Finally, let’s consider boxplots. Boxplots are commonly drawn with empty boxes, as in Figure 25.8. I prefer a light shading for the box, as in Figure 25.9. The shading separates the box more clearly from the plot background, and it helps in particular when we’re showing many boxplots right next to each other, as is the case in Figures 25.8 and 25.9. In Figure 25.8, the large number of boxes and lines can again create the illusion of background areas outside of boxes being actually on the inside of some other shape, just as we saw in Figure 25.1. This problem is eliminated in Figure 25.9. I have sometimes heard the critique that shading the inside of the box gives too much weight to the center 50% of the data, but I don’t buy that argument. It is inherent to the boxplot, shaded box or not, to give more weight to the center 50% of the data than to the rest. If you don’t want this emphasis, then don’t use a boxplot. Instead, use a violin plot, jittered points, or a sina plot (Chapter 9).