Multiple properties bar chart

Would love there to be a way to - essentially - merge two series into one e.g. I'm currently trying to show the percentage breakdown, and gross number on the same bar chart - I'm currently achieving it by making two series, one for percentage, one for gross number, a second Y axis showing the gross number - but what would be better is one series with multiple properties, with a Y axis for each property.

This would mean allowing custom settings for split axes ( #3897 ) and (maybe?) custom series for a Y axis ( #3242 )

:arrow_down: Please click the :+1: reaction instead of leaving a `+1` or `update?` comment

Start by reviewing the related issues #3897 and #3242, then examine how the current chart represents separate percentage and gross-number series with separate Y axes. Done means a single series can expose multiple properties, with an appropriate Y axis for each property and customizable split-axis settings.

Written by the indexing model from the issue text.