Which brings us back to the first subject of our series. Genome.gov publishes a quarterly summary graphic showing the loci of all the SNP-trait associations with p-values < 1.0 x 10-5, plotted as colored dots on a graphic representation of the human chromosome complement.
The problem here is that color semiology is not appropriate for such a large value range. We simply don’t do well differentiating that many different colors from a field of dots. Compounding the issue is the effect of the Gestalt Color Principle – our brains want to group together things with really similar colors, which can be useful in some instances, but here it just makes matters worse.
Color Perception Across Culture - The Berlin Kay Palette
In 1969, Brent Berlin and Paul Kay published a groundbreaking study of color perception across culture, in which they proposed that there were really 11 fundamental (or “focus”) colors that everyone could easily differentiate, most likely based on some underlying physiological or neurological principle. The Berlin-Kay palette was extended to include cyan by the visualization guru Colin Ware, giving us 12 colors that are reasonably safe to use for ordinal color semiology in infographics and data visualization. What do I mean by “ordinal” color semiology? Data dimensions that are sets of things (like categories, without quantitative interrelationships), rather than continuous, ordered, quantitative values, are ordinal. We can also use color for quantitative values – in fact, we can even split color into its three component subdimensions – hue,saturation and value – and use each of these to represent a separate quantitative dimension. Heatmaps and terrain relief are examples of such quantitative color semiology. We still have to be careful though, because we’re fairly bad at discerning specific quantitative values in a color (hue, saturation, or value) range.
The graphic we’re considering here, the authors are trying to use ordinal color semiology for a number of separate ordinal values. By now, you should understand why this is ideal. It’s nearly 10 times as many colors as the Berlin-Kay set.
The Solution - A Hybrid Semiology
So, how might we improve matters? One approach might be to employ a hybrid semiology – for instance, grouping the traits into manageable sets (with 12 or fewer sets in total) and encoding these sets with color semiology. Then, within each set, numbering the traits (numeric semiology). Let’s see how this might work, using chromosome 18 as a guinea pig. First, here’s what we’re starting with (excerpted from the original document):
Notice how your eyes and mind have to work to make sense of this, even though it’s just one chromosome from the whole diagram – you can do it, but it’s not intuitive or fast. Also, notice that the two Type 1 diabetes dots may actually look slightly different in color, due to their proximity to dots of different colors – this kind of color interaction is another hazard of using lots of different colors jumbled together to represent things. If there were only 12 well-differentiated colors on the diagram this would not be as big of a problem, but on the full diagram with all the colors, there are too many things that are “lavender-mauve-ish” – so these kinds of visual effects become meaningful.
Now, let’s rework it a bit by sorting our traits into categories, and assigning a “Berlin-Kay safe” color to all traits in the same category. Then we’ll number within each category and put the numbers on the dots.
Suddenly, you can find things! It works well in both directions – whether you start from the legend or from the loci on the chromosome. This solution will scale up to quite a large number of traits without losing its efficacy, as long as the number of categories stays at 12 or less.
Is this the only solution (or even the best) to this problem? Probably not. There are other possibilities as well – one approach would be to split the diagram intomultiples – duplicate copies of the whole diagram, broken out by some value, such as the categories assigned above (i.e., a diagram showing only cancers, another showing only cardiovascular loci, etc.). A possible disadvantage to this approach would be that you would no longer see the proximity of seemingly unrelated traits on the same chromosome, which might hinder insight into linkages, etc.
I hope this stimulates your thinking about choosing the right way to signify things. Please comment – do you see another way to approach this? Do you think I’m way off base (or right on)? Also, if you encounter any other charts, visualizations or infographics that you think could use a makeover, please send me a link and I’ll add them to the list for consideration.