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Theshared_yaxes parameter to make_subplots did not previously work onsecondary y-axes. It had no effect.

This PR fixes this, without affecting how the parameter is applied to normal plots (without secondary y-axes). I have added a simple test to show how the new code "matches" secondary y-axes. The sharing for secondary y-axes is applied right-to-left, in the same way that the sharing for primary y-axes is applied left-to-right (I believe this results in the clearest separation of tick marks when viewing the final plot).

I believe this is a bugfix/improvement. I'm not sure if it's a new feature worthy of any documentation.

I will add some images showing a before/after comparison in comments.