Let’s point out a few things.
First, the fill color scale has been carefully crafted to optimally show differences between countries.
Second, we are simultaneously using the highlighting technique to highlight the OPEC countries.
Finally, notice that we’re using the title to “tell a story” about the highlighted data.
All told, there is a lot going on in this example.
It’s a very interesting example of building higher-quality visuals in R. I also give them kudos for picking five colors which work for people with every kind of Color Vision Deficiency. H/T R-Bloggers