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.

首先查看 issue #1668、#5098 和 #5100，然后比较现有的图例和色条行为。定义颜色、大小、符号及其他视觉属性的独立图例应如何表示多个 trace，以及点击切换交互应如何工作。完成的标准是每个属性都可以拥有自己的交互式图例，而不会将其与按 trace 划分的图例混为一谈。