I have a couple of charts where I use both Y axes, but often need a 3rd axis for percentage metrics. Assuming percentages range from 0 to 100%, it would be useful to allow a graph to display that range over the full height of the panel - there would be no need for a scale, as that would be implicit (and in any case, the tooltip legend displays the value).
For example, an air conditioning system may want to display indoor temperature (F) on the left y-axis, power (kW) on the right y-axis, and indoor humidity, as a percentage, over the whole panel height.
Thanks,
Andrew