theta/r guides get theta/r theme elements

[teunbrand]

This PR to the RC aims to fix an internally discussed issue.

Briefly, it makes the theme settings that axes in coord_radial() listen in on more consistent with coord_polar() in that the theta guide will use x.bottom when theta = "x" and y.left when theta = "y".

Unfortunately due to default margin settings, this looks pretty bad for the r-axis when theta = "y".

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

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  guides(theta.sec = "axis_theta", r.sec = "axis") +
  theme(
    axis.line.x.bottom = element_line(colour = 'red'),
    axis.line.x.top = element_line(colour = "blue"),
    axis.line.y.left = element_line(colour = "green"),
    axis.line.y.right = element_line(colour = "orange")
  )

p + coord_radial(theta = "x", donut = 0.5)

p + coord_radial(theta = "y", donut = 0.5)

^{Created on 2024-02-06 with reprex v2.1.0}

[teunbrand]

As suggested, the r/theta positions get their own theme elements.
I didn't implement theta.sec and r.sec theme elements, but you can control this at the guide level anyway.

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2
ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  coord_radial(theta = "x", donut = 0.5) +
  theme(
    axis.line.theta = element_line(colour = 'red'),
    axis.line.r = element_line(colour = "green")
  ) +
  guides(
    theta.sec = guide_axis_theta(theme = theme(axis.line.theta = element_line(colour = "blue"))),
    r.sec = guide_axis(theme = theme(axis.line.r = element_line(colour = "orange")))
  )

^{Created on 2024-02-08 with reprex v2.1.0}

[thomasp85]

LGTM