Our last post generated much discussion around double axes. In this post, we take up Michael's suggestion of a scatter plot, and several suggestions to retain the original units.
The scatter plot in this case did not provide any insight, unfortunately. See below. It just highlighted the jerkiness in the data so we ended with much zig-zagging.
Retaining the original units is not advisable because those units were not comparable. In the following caricature, we show how to shape the axis to tell any story we want.
Panel plots are slightly better insofar as such mischief could be spotted by the amount of white space.
Another way to make the two data series comparable is to plot percentage change from year to year. This is similar to indexing, just the difference between annual change and cumulative change.