Axis caps

[teunbrand]

This PR aims to fix #4907.

Briefly, it adds a cap argument to guide_axis() that truncates the axis line to the breaks. There are 4 different options, presented below. Also TRUE/FALSE are shortcuts for "both"/"none".

I've thought about allowing user-defined coordinates as the capping points, either by function or input on data values, but that'd complicate the implementation quite a bit. I also thought that this is probably simple enough not to separate out to a separate guide.

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

ggplot(mtcars, aes(hp, disp)) +
  geom_point() +
  theme(axis.line = element_line()) +
  guides(
    x = guide_axis(cap = "both"),
    y = guide_axis(cap = "upper"),
    y.sec = guide_axis(cap = "lower"),
    x.sec = guide_axis(cap = "none")
  )

^{Created on 2023-04-26 with reprex v2.0.2}

[grantmcdermott]

+1ing as a user who has been looking forward to this capability for a while :-)

OTOH am I correct in thinking that, even with this PR and #4879, there still isn't a way to enable this capping behaviour globally? E.g. You still can't add guides as part of theme_set(), right?

[teunbrand]

That's right, I haven't added this as a theme option, but it might be something to consider. You can of course do the following:

capped_axes <- guides(
    x = guide_axis(cap = "both"),
    y = guide_axis(cap = "upper"),
    y.sec = guide_axis(cap = "lower"),
    x.sec = guide_axis(cap = "none")
  )
  
p1 <- ggplot(...) + ... + capped_axes
P2 <- ggplot(...) + ... + capped_axes
# etc.

[grantmcdermott]

Thanks, your suggestion is effectively what I do now (with your very nice ggh4x package). But I do think there’s additional value in make it “themeable”. Adding my support to your consideration.

[teunbrand]

I think it might make more sense to fold in #4271 first before adding this as a theme setting, that way you could set default guides in the theme.

[grantmcdermott]

SGTM, thanks Teun.

[thomasp85]

LGTM

[eejd]

I know this is merged, but I wanted to second that the option to switch guides capping from complete to truncated should be supported as a theme element / option as well. I have been waiting for this feature and have used various workarounds, but I realized recently working on a large project where I needed to change a lot of aesthetics across a large number of plots, the ability to reformat as much as possible via theme application is very useful.

[teunbrand]

The way guides are implemented, that is not an easy feature to incorporate.
The capping is calculated before axes get access to theme settings, so at that point it is too late to truncate.
At the point axes get access to theme settings, it no longer has access to the scales/coord to rescale or deform the outermost positions.