Setting panel sizes in theme()

[teunbrand]

This PR aims to fix #5338.

Briefly, the layout object gets a new method that sets the panel sizes.

The gist is that you can give a vector of units to theme(panel.widths) or theme(panel.heights) that then applies to each panel. It overrules any aspect ratios set in the coord or theme.

Somewhat special behaviour occurs if you give a unit of length 1, but you have multiple panels. In that case, the total panel area is set, rather than the size of individual panels.

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

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  facet_grid(~ drv)

# Square 2x2cm panels
p + theme(
  panel.widths = unit(c(2, 2, 2), "cm"),
  panel.heights = unit(2, "cm")
)

# Total width is 6cm, so every panel is somewhat narrower than 2cm
p + theme(
  panel.widths = unit(6, "cm"),
  panel.heights = unit(2, "cm")
)

^{Created on 2024-09-09 with reprex v2.1.1}

I'm not quite sure whether the layout is the correct place for the method. It can also belong to the facet class, for example.

[teunbrand]

Currently interacts with {patchwork} like so:

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

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point()

p1 <- p + theme(panel.widths = unit(6, "cm"))
p2 <- p + theme(panel.widths = unit(4, "cm"))

Patchwork just ignores the sizes normally:

p1 / p2

But can preserve space when the plots are freed. It'd be neat if we could align this on the left or right side of the plot.

free(p1) / free(p2)

^{Created on 2024-09-09 with reprex v2.1.1}

[thomasp85]

With dev patchwork it now works in the sense that the dimensions gets adopted from the panel with the largest dimensions (if set in absolute units)

There is really no way to accommodate conflicting sizes while keeping the alignment model of patchwork so something got to give