Introducing multi-legend feature from plotly.js v2.22 will lead to breaking changes

[kMutagene]

Just adding this as an issue to the milestone so it is immediately visible.

This means that all new upstream feature additions will be on v5, which will then most likely have some preview versions again until it is completely up-to date with plotly.js

[kMutagene]

still to-do:

• dedicated legend field on all trace constructors
• tests