Add scale_color_units to allow for units in continous color scales.

[ekatko1]

It would be great if continuous color scales could also have units.

Example:

library(ggplot2)
library(units)
library(ggforce)

# Plot looks great without units:
ggplot(iris, aes(x=Sepal.Length, y=Sepal.Width, color = Petal.Length)) + 
  geom_point()

# Adding units to the dataset
iris$Sepal.Length =  set_units(iris$Sepal.Length, cm)
iris$Sepal.Width  =  set_units(iris$Sepal.Width,  cm)
iris$Petal.Length =  set_units(iris$Petal.Length, cm)
iris$Petal.Width  =  set_units(iris$Petal.Width,  cm)

ggplot(iris, aes(x=Sepal.Length, y=Sepal.Width, color = Petal.Length)) + 
  geom_point()

The ggplot with units in the color aesthetic causes the following error:

Error in Ops.units(x, range[1]) : 
  both operands of the expression should be "units" objects

Thanks :)