You're quite right that we're double-coding along the X-axis (and the Y-axis as well to a lesser extent). The introduction and the "How to read this chart" tool

You're quite right that we're double-coding along the X-axis (and the Y-axis as well to a lesser extent). The introduction and the "How to read this chart" tooltip should help explain that, as should the year legend within each mini-chart ... but if you have a concrete idea about where we could place additional explanation to make it clearer, let me know.