Move {mgcv} to suggests

[teunbrand]

This PR fixes #5986 and closes #5985.

Briefly, when method = "gam" and {mgcv} is not installed, we fallback to method = "lm" with a warning.
Needed a mock test to verify correct message is thrown and add one skip step.

Please note that downstream dependencies that use geom_smooth(method = "gam") will need to add {mgcv} to their dependencies.

[clauswilke]

@teunbrand I believe this is a reasonable solution but just wanted to raise one potential issue: I believe once mgcv is in suggests it no longer gets installed automatically when people install ggplot2. This means most people will likely not have it installed going forward. I don't know what to do about this, just one of the annoyances of the R ecosystem that I've run into a couple of times. (I tell my students to install packages X, Y, and Z, and then key functionality is missing because some dependencies have been moved into suggests and no longer are installed automatically.)

[yutannihilation]

This means most people will likely not have it installed going forward.

I think it's a good point. I believe rlang::check_installed(), which interactively asks the user to install the package, should solve this as we do in several places.

https://github.com/search?q=repo%3Atidyverse%2Fggplot2%20check_installed&type=code

[clauswilke]

@yutannihilation Ah, good point. Yes, this seems like the way to go.

Edit: Oh, but then there's no fallback in case the user doesn't want to install the package, right? It's just a hard check.

[teunbrand]

Yeah the lack of fallback is why I didn't use that initially, but giving it a 2nd thought we could probably wrap it in a try_fetch() block to set a fallback.

[clauswilke]

Thinking some more about it, this can get rather cumbersome quickly if somebody can't install mgcv for some reason and gets prompted to do it every time they try to make a plot.

Here is an alternative to consider: ggplot could check on startup (library load) what packages are installed/missing and on occasion (once per session, once per week, whatever) print a message that lists suggested but not installed packages that would enable additional functionality.

Such a function could also not be run automatically but be available for manual checking.

[teunbrand]

this can get rather cumbersome quickly if somebody can't install mgcv for some reason and gets prompted to do it every time they try to make a plot.

This would only trigger when method = "gam", either explicitly or by default when nrow(data) > 1000. If they don't want to be prompted about this, they could set the method argument to something else in order to opt-out these prompts.

[clauswilke]

I see. Maybe mention this in the message that is generated?

[teunbrand]

wrap it in a try_fetch() block

This turns out trickier than anticipated since check_installed() has some restart condition that is too slippery for me to catch in a try_fetch() block.

ggplot could check on startup (library load) what packages are installed/missing

It would be handy to have a ggplot2::install_suggested() function to install the functional dependencies.

[teunbrand]

This is what happens now if {mgcv} is not installed in non-interactive versions.

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

rlang::is_installed("mgcv")
#> [1] FALSE

ggplot(mpg, aes(displ, hwy)) +
  geom_smooth(method = "gam")
#> The `method` was set to "gam", but mgcv is not installed.
#> ! Falling back to `method = "lm"`.
#> ℹ Change the `method` argument to silence this message.
#> `geom_smooth()` using formula = 'y ~ x'

^{Created on 2024-07-11 with reprex v2.1.0}

In interactive versions, you'll be prompted to install.
If you refuse, you get the same as the example above.
If you install {mgcv}, you continue with method = "gam" as per usual.

[clauswilke]

Does the message also appear when people don't set the method explicitly but just use a large dataset with >1000 points? If yes then I'd suggest to say something like "Install mgcv or change the method argument ..."

[teunbrand]

Thanks for the suggestion Claus, it now reads:

The method was set to "gam", but mgcv is not installed.
! Falling back to method = "lm".
ℹ Install mgcv or change the method argument to resolve this issue.

[thomasp85]

LGTM