[bradford]

I was coming here to say something similiar.

The article only shows examples of dual axis charts where line series are used for both axes. This will clearly cause confusion (especially when tooltips are not available).

I've generally found that when displaying a percentage, it is helpful to show the individual counts for numerator/denominator. I believe that showing percentage as a line series on one axis, and raw counts, represented as a column on the other axis, can be a helpful visual.