Deprioritise annotations in deriving labels

[teunbrand]

This was first suggested in tidyverts/feasts#166 (comment).

When you use an annotation you want to use as the background, you use that annotation before the real 'data' layers. For example, to highlight the 70s you can use:

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

ggplot(economics, aes(date, unemploy)) +
  annotate(
    "polygon",
    x = as.Date(c("1970-01-01", "1970-01-01", "1980-01-01", "1980-01-01")),
    y = c(Inf, -Inf, -Inf, Inf),
    alpha = 0.25
  ) +
  geom_line()

^{Created on 2025-01-31 with reprex v2.1.1}

However the default label is now x and not date and the question is whether we should deprioritise annotations in deriving automatic labels? Currently, there is nothing distinguishing an annotation layer from a regular layer beyond the constructor.

[clauswilke]

I think the general rule would be to use global aesthetics before layer-specific aesthetics. Not sure how easy that would be to implement. This might require a complete rethinking of when and how axis titles are defined.

[teunbrand]

We explicitly avoided that approach based on #5894.
It does not mesh well with automatically extracting the label attribute, as the global mapping is never evaluated and hence the attribute cannot be retrieved.

[clauswilke]

I see. I guess there's always going to be edge cases that are difficult to catch. In the end, people can always override labels manually if they need to.

[teunbrand]

True enough. Just to repeat a bit of reasoning from the original discussion, an argument why annotations should be skipped is that the annotation's aesthetics is not (directly) 'mapped data' unlike regular layers. In addition, it will always give 'x' as the default label for the x aesthetic because that is how the column names are constructed internally, so it will never be informative for global data.

[teunbrand]

Arguably we also shouldn't let expand_limits() affect labels. In the example below, I think displ is the more appropriate x-axis title.

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

ggplot(mpg, aes(displ, hwy)) +
  expand_limits(x = c(0, 10)) +
  geom_point()

^{Created on 2025-02-05 with reprex v2.1.1}