Fix colourbar's ticks.length argument not working

[teunbrand]

I became aware that the ticks.length argument of guide_colourbar() wasn't working as intended, so this PR fixes that problem.
I've including this argument in an already existing test so that we might know if this breaks again in the future.

Small demonstration of setting the tick length:

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

ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(colour = cty)) +
  guides(colour = guide_colourbar(ticks.length = unit(0.5, "npc")))

^{Created on 2023-11-20 with reprex v2.0.2}

[thomasp85]

What issue does this solve? Can you elaborate since you don't link to an issue?

[teunbrand]

Apologies for not filing an issue first, which I should have.

The issue is that the ticks.length argument was ignored. Here is the same reprex on the current main branch, where you can see that the ticks don't follow the length set in the ticks.length argument and retain the default unit(0.2, "npc") tick length:

library(ggplot2)
packageVersion("ggplot2") # current main
#> [1] '3.4.4.9000'

ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(colour = cty)) +
  guides(colour = guide_colourbar(ticks.length = unit(0.5, "npc")))

^{Created on 2023-11-20 with reprex v2.0.2}

[thomasp85]

LGTM

[teunbrand]

Thanks for the review!