scale_color_stepsn() will not accept expression vector for scale labels

[palderman]

An error is generated when attempting to specify color scale labels as a plotmath expression vector. The error is also generated when using scale_fill_stepsn(). I expected the color scale labels to be rendered using plotmath.

Here is code to reproduce the bug:

library(ggplot2)
data.frame(x = 1:5, y = 1:5) |>
  ggplot(aes(x = x, y = y, color = y))+
  geom_text(label = parse(text = paste0("x^", 1:5)))+
  scale_color_stepsn(breaks = 1:5,
                                  colors = c("red", "orange", "yellow","green", "blue"),
                                  labels = parse(text = paste0("x^", 1:5)))
#> Error in `vec_size()`:
#> ! `x` must be a vector, not an expression vector.

^{Created on 2024-07-17 with reprex v2.1.0}

[palderman]

I am able to achieve the desired result by providing a list-vector of expressions instead. If this is the intended behavior then I think the help entry should be updated accordingly. See reprex:

library(ggplot2)
data.frame(x = 1:5, y = 1:5) |>
  ggplot(aes(x = x, y = y, color = y))+
  geom_text(label = parse(text = paste0("x^", 1:5)))+
  scale_color_stepsn(breaks = 1:5,
                     colors = c("red", "orange", "yellow","green", "blue"),
                     labels = as.list(parse(text = paste0("x^", 1:5))))
#> Warning in is.na(x): is.na() applied to non-(list or vector) of type
#> 'expression'

^{Created on 2024-07-17 with reprex v2.1.0}

[teunbrand]

Thanks for the report! I'm suspecting that guide_coloursteps() probably doesn't convert vectors-expressions to list-expressions. The {vctrs} package can cope with list-expressions but not vector-expressions, which is what causes the error. This shouldn't be redocumented but fixed in the guide.

[teunbrand]

In addition, the following should also work but doesn't:

library(ggplot2)

data.frame(x = 1:5, y = 1:5) |>
  ggplot(aes(x = x, y = y, color = y))+
  geom_text(label = paste0("x^", 1:5), parse = TRUE)+
  scale_color_stepsn(
    breaks = 1:5,
    colors = c("red", "orange", "yellow","green", "blue"),
    labels = parse(text = paste0("x^", 1:5)),
    guide = guide_bins()
  )
#> Error in `scale_color_stepsn()`:
#> ! `breaks` and `labels` have different lengths.

^{Created on 2024-07-18 with reprex v2.1.1}