How to add a color formatting option for each legend item

[WouterBo]

In the standard Line Chart visual, the formatting options (under Data Colors) show a color selector for each item in the legend. It appears to be dynamic, i.e. the formatting pane dynamically adjusts to the number of legend items present.

How do I recreate this behaviour in my own visual?

[WouterBo]

https://community.powerbi.com/t5/Developer/Custom-Object-Formatting/m-p/988078