Add value matching to breaks in manual_scale

[idno0001]

Fixes #2429.

As noted in the issue, currently values can be matched with a named vector. The below examples show (1) this as a baseline, (2) how behaviour changes as breaks are added, and the new behaviour (3) when values is not a named vector.

library(ggplot2)

df <- data.frame(x = c("data_red", "data_black"))

p <- ggplot(df, aes(x, 1, fill = x)) + 
  geom_col()

# Named values, no breaks
p + scale_fill_manual(values = c("data_red" = "red", "data_black" = "black"))

# Named values with breaks
p + scale_fill_manual(values = c("data_red" = "red", "data_black" = "black"),
                      breaks = c("data_red", "data_black"))

# New behaviour: values matched to breaks
p + scale_fill_manual(values = c("red", "black"),
                      breaks = c("data_red", "data_black"))

^{Created on 2019-10-19 by the reprex package (v0.3.0)}

[clauswilke]

Just to be sure this actually works as it should, could you provide a reprex where the data values are not identical to the color names? E.g., use this as your data:

df <- data.frame(x = c("data_red", "data_black"))

[idno0001]

That's a good point; I've updated the reprex as you suggested.

[clauswilke]

I've looked over the patch. It looks good to me, but we should get a second opinion from somebody else on the team.

A few additional items we will need:

• A news item that describes the new behavior. Importantly, this can change the visual output for some plots where people have relied on the previous, unordered behavior. (Not sure how to assess how big of a problem this will be.)
• A unit test for the new behavior. Probably something similar to this one:
  

  ggplot2/tests/testthat/test-scale-manual.r

  Lines 3 to 7 in 115c396

  	test_that("names of values used in manual scales", {
  	s <- scale_colour_manual(values = c("8" = "c","4" = "a","6" = "b"))
  	s$train(c("4", "6", "8"))
  	expect_equal(s$map(c("4", "6", "8")), c("a", "b", "c"))
  	})

[clauswilke]

Oh, also, put "Fixes #2429." into your next commit, so this is properly recorded in the git log.

[idno0001]

I've added the news item and a couple of unit tests. Thanks for the feedback again!

[clauswilke]

Looks good. A few more minor changes are needed.

[idno0001]

All done. Give me a shout if there's anything else.

[clauswilke]

Generally this looks good to me.

@thomasp85 Could you take a look also?

[thomasp85]

Thank you for your contribution!