Better legend items visual for pie-like traces with same labels / different colors

[etpinard]

See https://codepen.io/etpinard/pen/arVVpg

for pie traces, funnelarea traces behave similarly.

Potential solution:

• split the legend item square into two halves (in general into N parts), one half with the pie color, the other with funnelarea color

More info: #3876 (comment)

[archmoj]

cc #2858.

[etpinard]

Thanks for spotting #2858 @archmoj - I'll merge this ticket in #2858