Extracting guide data

[teunbrand]

This PR is the updated version of #5096.
It aims to address issues such as #5004.

Briefly, it adds a guide_data() function to programmatically access values and labels of axes and legends. More motivation is mentioned in #5096 (comment).

Here is what is different relative to the previous PR:

• This PR uses the new guide system.
• No kerfuffle with CoordSf, as that coord uses the new guide system now.
• If there is only 1 matching guide, it outputs a data.frame directly instead of a list.

Examples:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

# A typical plot
p <- ggplot(mtcars) +
  aes(mpg, disp, colour = drat, size = drat) +
  geom_point() +
  facet_wrap(vars(cyl), scales = "free_x")

# Getting guide data for position aesthetic in particular panel
guide_data(p, "x", i = 1, j = 2)
#>           x .value .label y
#> 1 0.0959596     18     18 0
#> 2 0.3484848     19     19 0
#> 3 0.6010101     20     20 0
#> 4 0.8535354     21     21 0

# Getting guide data for a legend
guide_data(p, "size")
#>       size .value .label
#> 1 2.662822    3.0    3.0
#> 2 3.919819    3.5    3.5
#> 3 4.779645    4.0    4.0
#> 4 5.477285    4.5    4.5

# If guides are merged, `guide_data()` reports merged guide data
guide_data(p + guides(colour = "legend"), "size")
#>    colour .value .label     size
#> 1 #1A3855    3.0    3.0 2.662822
#> 2 #28557C    3.5    3.5 3.919819
#> 3 #3874A5    4.0    4.0 4.779645
#> 4 #4894D0    4.5    4.5 5.477285

^{Created on 2023-06-26 with reprex v2.0.2}

[teunbrand]

I thought it was cleaner to early-exit out of guides$build() than to re-implement the build code.

[teunbrand]

@cpsievert since {plotly} extracts guide information, and this is the intended use-case for this PR, do you think this PR extracts enough information from guides to be useful?

[cpsievert]

Seems good to me, thanks!

[teunbrand]

Closing this PR in favour of #5506.