Dual Axes, Not Dueling

I'm the author of this essay. I design charts and host tours of my library of rare information graphics in San Francisco. I also work on dbt Charts, which is where this essay started: we added a warning that fires when a chart puts two measures on independent y-axes. The case against dual axes is old but I thought it deserved some nuance. There are different kinds of dual axes (competing, mirrored, translation). I also wanted to add an exchange rate metaphor: put two scales on one plot and some number of right-axis units always equals one left-axis unit, whether anyone states it or not. I took inspiration from Lisa Charlotte Muth (Datawrapper), Nathan Yau (Flowing Data) and historic writing and examples, especially Karl Karsten's 'Charts and Graphs' (1923) and some later William Playfair (1820s). I'd love to see any dueling-axis chart you think earns its place.