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.

Inizia esaminando le issue #1668, #5098 e #5100, quindi confronta il comportamento esistente delle legende e delle barre dei colori. Definisci come legende separate per colore, dimensione, simbolo e altri attributi visivi debbano rappresentare più tracce e come debba funzionare l'interazione di attivazione e disattivazione tramite clic. Il lavoro è completato quando ogni attributo può avere la propria legenda interattiva senza confonderla con le legende per traccia.