Tertiary Y Axis.

I once had a page on this site that showed how to generate additional axes in a chart. With the technique you could go beyond primary and secondary axes, to tertiary and quaternary axes, and even more.

Given that charts with just primary and secondary axes can be confusing, a tertiary axis is just overkill. The chart shown above is cluttered and confusing, even using a custom color scheme to help identify each series with its corresponding axis. You can read further rationale in the following articles, for removing secondary axes from charts and leaving the user with simple primary axes, in the following articles:

Dual-Scaled Axes in Graphs: Are They Ever the Best Solution? by Stephen Few, Perceptual Edge.
Secondary Axes in Charts.
Secondary Axes that Work - Proportional Scales.

I propose using Panel Charts as a substitute for charts with confusing multiple axes. Here are some tutorials and examples of panel charts:

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