Use symbol_color variable for legend

[jvdkwast]

Nyall Dawson showed in his demo on YouTube today that you can use the styling colours of a layer for label font colours by using @symbol_color as a variable. I wonder if it would be possible to implement that for the colours of DataPlotly legends.

It's mentioned here in the video (50m07s)

[nyalldawson]

Can you give some more example of how this would be used? I'm struggling to understand the use case

[jvdkwast]

Here's a video about the use case: https://youtu.be/QbKmJ7Dg3o0

[ambarja]

Here's another video with another expression. https://youtu.be/pGnZdj4W8ew?t=235 n-n

[chrisgraf]

@nyalldawson : The usecase would be the synchronization of the symbology color and the associated color in the diagram.

[katagen]

This feature will be very useful!

[ghtmtt]

done in #288