Add functions for checking plot labels

[rossellhayes]

Adds function get_labels() for listing (a subset of) the labels of a plot, and uses_labels() for checking if labels match their expected values.

library(ggcheck)
library(ggplot2)

p <- ggplot(mtcars, aes(x = wt, y = mpg, color = cyl)) +
  geom_point() +
  labs(x = "weight", y = "MPG", color = NULL)

get_labels(p)
#> $x
#> [1] "weight"
#> 
#> $y
#> [1] "MPG"
#> 
#> $colour
#> NULL

get_labels(p, c("x", "y"))
#> $x
#> [1] "weight"
#> 
#> $y
#> [1] "MPG"

uses_labels(p, x = "weight")
#> [1] TRUE

uses_labels(p, color = NULL)
#> [1] TRUE

# If multiple arguments are given, returns TRUE only if all labels match
uses_labels(p, x = "weight", color = NULL)
#> [1] TRUE

^{Created on 2021-11-12 by the reprex package (v2.0.1)}

[deirdrefrancks]

Looks good to me!

[gadenbuie]

Let's use rlang or refactor this into a helper function or intermediate variables

[rossellhayes]

Addressed in 7a5677c

[gadenbuie]

LGTM!