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.

まず issue #1668、#5098、#5100 を確認し、既存の凡例とカラーバーの動作を比較します。色、サイズ、シンボル、その他の視覚属性について個別の凡例が複数のトレースをどのように表現すべきか、またクリックによる表示切り替えのインタラクションをどのように機能させるべきかを定義します。各属性がトレースごとの凡例と混同されることなく、それぞれ独自のインタラクティブな凡例を持てるようになれば完了です。