tl;dr: The functionality shown in this post is now on the ggnewscale package! 📦. You can find the original code in this gist. A somewhat common annoyance for some ggplot2 users is the lack of support for multiple colour and fill scales. Perusing StackOverflow you can find many questions relating to this issue: Unfortunately, this deluge of questions is met with a shortage of conclusive answers, most of them being some variation of “you can’t, but here’s how to hack it or visualise the data differently”.
As an atmospheric scientists, a lot of my research consists on plotting and looking at global fields of atmospheric variables like pressure, temperature and the like. Since our planet is a sphere (well, almost), it is unbound and so longitude is a periodic dimension. That is, to the right of 180°E you go back to 180°W. But ggplot2 and other plotting systems, for the most part, assume linear dimensions.
For a while now I’ve been thinking that, yes, ggplot2 is awesome and offers a lot of geoms and stats, but it would be great if it could be extended with new user-generated geoms and stats. Then I learnt that ggplot2 actually has a pretty great extension system so I could create my own geoms I needed for my work or just for fun. But still, creating a geom from scratch is an involved process that doesn’t lend itself to simple transformations.