Allows units objects to define ggplot2 aesthetics other than x and y

[agila5]

Dear all,

I decided to create this issue since I would like to ask a question (and probably request a new feature). I noticed that after #164 we can use units objects to define the x and y axis in a ggplot2 thanks to the implementation of scale_x_units and scale_y_units. For example:

library(ggplot2)
library(units)
#> udunits database from C:/Users/user/AppData/Local/R/win-library/4.3/units/share/udunits/udunits2.xml

dat <- data.frame(
  x = set_units(1, m), 
  y = set_units(1, m), 
  z = 1
)

ggplot(dat) + 
  geom_point(aes(x, y, col = z))

However, as soon as I specify that the variable z is recorded with a given unit of measurement, the same code fails

dat$z <- set_units(dat$z, "m")
ggplot(dat) + 
  geom_point(aes(x, y, col = z))
#> Error in Ops.units(x, range[1]): both operands of the expression should be "units" objects

^{Created on 2024-05-25 with reprex v2.0.2}

The problem is that something like scale_colour_units is missing, right? Are there any particular difficulties in implementing it? I'm not really familiar with the ggplot2 internals but, if that's the case, I can try creating a PR sometimes during summer.

Session info

sessioninfo::session_info()
#> ─ Session info ───────────────────────────────────────────────────────────────
#>  setting  value
#>  version  R version 4.3.1 (2023-06-16 ucrt)
#>  os       Windows 11 x64 (build 22631)
#>  system   x86_64, mingw32
#>  ui       RTerm
#>  language (EN)
#>  collate  English_United Kingdom.utf8
#>  ctype    English_United Kingdom.utf8
#>  tz       Europe/Rome
#>  date     2024-05-25
#>  pandoc   3.1.1 @ C:/Program Files/RStudio/resources/app/bin/quarto/bin/tools/ (via rmarkdown)
#> 
#> ─ Packages ───────────────────────────────────────────────────────────────────
#>  package     * version    date (UTC) lib source
#>  cli           3.6.2      2023-12-11 [1] CRAN (R 4.3.3)
#>  colorspace    2.1-0      2023-01-23 [1] CRAN (R 4.3.1)
#>  curl          5.2.1      2024-03-01 [1] CRAN (R 4.3.3)
#>  digest        0.6.35     2024-03-11 [1] CRAN (R 4.3.3)
#>  dplyr         1.1.4      2023-11-17 [1] CRAN (R 4.3.1)
#>  evaluate      0.23       2023-11-01 [1] CRAN (R 4.3.3)
#>  fansi         1.0.6      2023-12-08 [1] CRAN (R 4.3.2)
#>  farver        2.1.2      2024-05-13 [1] CRAN (R 4.3.1)
#>  fastmap       1.1.1      2023-02-24 [1] CRAN (R 4.3.1)
#>  fs            1.6.4      2024-04-25 [1] CRAN (R 4.3.3)
#>  generics      0.1.3      2022-07-05 [1] CRAN (R 4.3.1)
#>  ggplot2     * 3.5.1.9000 2024-05-25 [1] Github (tidyverse/ggplot2@28aec3a)
#>  glue          1.7.0      2024-01-09 [1] CRAN (R 4.3.2)
#>  gtable        0.3.5      2024-04-22 [1] CRAN (R 4.3.1)
#>  highr         0.10       2022-12-22 [1] CRAN (R 4.3.1)
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#>  labeling      0.4.3      2023-08-29 [1] CRAN (R 4.3.1)
#>  lifecycle     1.0.4      2023-11-07 [1] CRAN (R 4.3.2)
#>  magrittr      2.0.3      2022-03-30 [1] CRAN (R 4.3.1)
#>  munsell       0.5.1      2024-04-01 [1] CRAN (R 4.3.3)
#>  pillar        1.9.0      2023-03-22 [1] CRAN (R 4.3.1)
#>  pkgconfig     2.0.3      2019-09-22 [1] CRAN (R 4.3.1)
#>  purrr         1.0.2      2023-08-10 [1] CRAN (R 4.3.1)
#>  R.cache       0.16.0     2022-07-21 [1] CRAN (R 4.3.1)
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#>  R6            2.5.1      2021-08-19 [1] CRAN (R 4.3.1)
#>  Rcpp          1.0.12     2024-01-09 [1] CRAN (R 4.3.1)
#>  reprex        2.0.2      2022-08-17 [1] CRAN (R 4.3.1)
#>  rlang         1.1.3      2024-01-10 [1] CRAN (R 4.3.1)
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#>  rstudioapi    0.16.0     2024-03-24 [1] CRAN (R 4.3.1)
#>  scales        1.3.0      2023-11-28 [1] CRAN (R 4.3.1)
#>  sessioninfo   1.2.2      2021-12-06 [1] CRAN (R 4.3.1)
#>  styler        1.10.2     2023-08-29 [1] CRAN (R 4.3.1)
#>  tibble        3.2.1      2023-03-20 [1] CRAN (R 4.3.1)
#>  tidyselect    1.2.0      2022-10-10 [1] CRAN (R 4.3.1)
#>  units       * 0.8-5.4    2024-04-27 [1] Github (r-quantities/units@56c0b9d)
#>  utf8          1.2.4      2023-10-22 [1] CRAN (R 4.3.1)
#>  vctrs         0.6.5      2023-12-01 [1] CRAN (R 4.3.2)
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#>  xfun          0.43       2024-03-25 [1] CRAN (R 4.3.3)
#>  xml2          1.3.6      2023-12-04 [1] CRAN (R 4.3.2)
#>  yaml          2.3.8      2023-12-11 [1] CRAN (R 4.3.2)
#> 
#>  [1] C:/Users/user/AppData/Local/R/win-library/4.3
#>  [2] C:/Program Files/R/R-4.3.1/library
#> 
#> ──────────────────────────────────────────────────────────────────────────────

[edzer]

That would be great, and have the legend title z [m]!