This is part of a series of posts about the 'little of visualisation design', respecting the small decisions that make a big difference towards the good and bad of this discipline. In each post I'm going to focus on just one small matter - a singular good or bad design choice - as demonstrated by a sample project. Each project may have many effective and ineffective aspects, but I'm just commenting on one.
The 'little' of this next design concerns colour choices for divergent scales, ie. when you are showing quantitative scales in two directions either side of a pivot/breakpoint (often zero or an average). In this analysis by the Washington Post, we see a choropleth map showing the gender gap in adult employment rates for children of low-income families, based on the county where they grew up.
The little issue here concerns the respective shade of purple/green used for the extremes - the highest value colour bands - which, inevitably, tend to represent the most interesting insights of such analysis. Whilst the underlying choice of a purple > green colour scheme makes loads of sense (and importantly will be colour-blindness friendly) the darkest shades of the purple and green are very similar upon scanning the map.
Sure, you *can* distinguish between these shades but it is perhaps more effort for the eye to perform this separation, quickly, than it needs to be. Maybe making the second darkest shade of purple/green representative of the highest value banding would make it a little easier.
Agreed that there are some color issues that make it a little tougher to read (the +/- 9 to 15 range looks off to my eye) but, wow, it is so refreshing to see they didn’t use blue and pink.
Like it!
You could read this on how to improve colors: vis4.net/blog/posts/avoid-equidistant-hsv-colors/
Also, I’d like to see the extremes really stand out. So, the average would be more/less gray and the extremes saturated colors.
I’m wondering if the extremes are that dark because if they used a more limited palette, it would be harder to distinguish between individual categories? If that was hypothetically the case, which would the better trade off be in your opinion Andy?
Thanks for all the comments. (Yes, Anthony, agreed re. the lack of blue/pink
Matt – yes there’s always a trade-off that has to be finely judged and know for sure the creator (Christopher Ingraham, who is exceptional) would have thought carefully about the balance to strike. I suspect the palette was generated from Colorbrewer (http://colorbrewer2.org/) as this is a presented option for diverging scheme. So it is based on good principles for sure! Personally I just feel the extremes are always likely to be more interesting than the low values so should be more worthy of having the eye directed to them as extremes but equally clearly belonging to their respective categorical groupings. This type of decision I describe as being an editorial one. There’s no right or wrong choice, it is simply based on what you most accessibly want your reader’s eyes to pounce on.
Awesome, thanks for your thoughts Andy 🙂