allow multiple legends by size, symbol, color etc

Breaking off of https://github.com/plotly/plotly.js/issues/1668 ...

Multiple users have requested the ability to have per-visual-attribute legends, similar to systems like ggplot2, Tableau etc so one legend for discrete color, one for size, one for symbol and so on. This is distinct from simply having multiple legends where each trace has a single legend item because in this concept, each trace would cause a legend item to appear in each legend, and furthermore, each legend item could refer to multiple traces. It's more similar to our existing coloraxis/colorbar concept than to our legend concept.

The major challenge for us is interactivity: it's relatively easy in a static tool like ggplot2 or matplotlib to just draw a second legend, but our legends support click-to-toggle etc (although our colorbars don't, today, and should!) and this would require a fairly big engineering lift.

Commencez par examiner les issues #1668, #5098 et #5100, puis comparez le comportement existant des légendes et des barres de couleur. Définissez comment des légendes distinctes pour la couleur, la taille, le symbole et les autres attributs visuels doivent représenter plusieurs traces, ainsi que le fonctionnement de l’interaction d’activation et de désactivation par clic. La tâche est considérée comme terminée lorsque chaque attribut peut avoir sa propre légende interactive sans la confondre avec les légendes par trace.