Feature request: add label_colour as an aesthetic

[MattCowgill]

Hello, Thank you for a very useful package.

It would be handy to be able to map label.colour to the data as an aesthetic. As an example:

library(ggplot2)
library(ggforce)

ggplot(mtcars, aes(x = wt, y = mpg, col = factor(cyl))) +
  geom_point() +
  ggforce::geom_mark_ellipse(aes(label = cyl)) 

^{Created on 2021-06-11 by the reprex package (v2.0.0)}

Here it would be useful to colour the 4, 6, and 8 labels the same as the points and ellipses, by mapping them to the data.

We can colour all annotations:

library(ggplot2)
library(ggforce)

ggplot(mtcars, aes(x = wt, y = mpg, col = factor(cyl))) +
  geom_point() +
  ggforce::geom_mark_ellipse(aes(label = cyl),
                             label.colour = "red") 

^{Created on 2021-06-11 by the reprex package (v2.0.0)}

But label.colour is not implemented as an aesthetic:

library(ggplot2)
library(ggforce)

ggplot(mtcars, aes(x = wt, y = mpg, col = factor(cyl))) +
  geom_point() +
  ggforce::geom_mark_ellipse(aes(label = cyl,
                                 label.colour = cyl)) 
#> Warning: Ignoring unknown aesthetics: label.colour

^{Created on 2021-06-11 by the reprex package (v2.0.0)}

Making label.colour an aesthetic would be a really useful addition to the package.

[rjake]

I'm also looking for this...

[stragu]

I agree this would be very handy to increase readability of all `geom_mark_*() functions when there are many categories or when they inconveniently overlap.

I think con.colour could similarly be made an aesthetic: I can see people preferring colouring the connector rather than the text of the label.

[thomasp85]

It is unfortunately not as simple as it sounds, since label.colour (and con.colour) would need their own scales as well etc and it quickly becomes unwieldy.

How would people feel about a sentinel like 'inherit' that would indicate the colour coming from the main colour aesthetic? It is also not straightforward to make work, but is probably a better path to take

[stragu]

I think 'inherit' would be sufficient as I don't expect it would be particularly popular to colour the connectors and labels with a scale different to the ellipses'.

[MattCowgill]

I agree 'inherit' would do the job! Thank you @thomasp85