See Misleading Y-Axis for EmonCMS.org for a related discussion.
also see Looking for a "dashboard" solution - #15 by GeorgeB for an example of alternative solution using the trace colors for the y-scale text, probably not the best idea to use this specifically but it’s food for thought eg colour the left and right axis red and blue and use those same colours for the legend text so that each entry is either blue or red text to indicate left or right.