Add last grey/gray spelling combination

[thomasp85]

This is not really the best, and I'm open to alternatives.

ggplot2 originally had scale_colour_grey(), and scale_color_grey(), with the former mixing up English/American spelling of colour and grey. A recent PR added scale_color_gray() which makes the tests fail as it looks for whether all English colour scales have an American equivalent. This PR adds scale_colour_gray() which fixes the test, but leaves a sour taste in my mouth. Dream scenario would be to only have scale_colour_grey() and scale_color_gray() as those names would be internally consistent, but I don't think we can remove the original mixed one as it would cause major breakage...

The alternative is to change the tests to remove one of the versions but that seems almost just as bad...

Thoughts

[hadley]

Why don't we just remove scale_color_gray()?

[clauswilke]

Counter-argument: I can never remember whether it's grey or gray and use them interchangingly.

[yutannihilation]

I too can't remember it, but always use tab completion so it's better if there are fewer candidates. (But I have no strong opinion here)

[clauswilke]

I'm approving this but don't have a strong opinion one way or another.

[thomasp85]

@hadley, it was recently added because it was lacking and US spelling users requested it

[yutannihilation]

@hadley
A bit more context is that you indicated that adding scale_color_gray is "a good idea," which aren't implemented just because we don't have the development bandwidth. This was the reasoning of the PR (#4299).

This seems like a good idea, but unfortunately it's not high enough on my priority list that I will implement it myself. However, I'd be happy to review a PR.
#1901 (comment)

[hadley]

@yutannihilation that's just my standard response. I wouldn't read anything into it.

I think reverting #4299 is the simplest fix — scale_color_gray() (no matter how you spell it) is relatively rarely used, and the spelling difference is small enough that I don't think it's really causing any problems.

[yutannihilation]

I see... Yeah, I personally agree with reverting it.

[clauswilke]

Fine with reverting also. Maybe I'll finally learn the correct spelling. :-)

[hadley]

Its A for America (grAy) and E for England (grEy) 😄

[clauswilke]

And what do they use in Australia?

[thomasp85]

ok, I'll revert instead