This is really nice! Am I correct that the radius axis angle provided to r_axis_inside is not calculated relative to the start point? I created a polar plot with a 360 degree x-axis limit. I chose a different start point and wanted to have the radial axis at 45 degrees, but I had to input 135 degrees instead, which is maybe counter intuitive. Also, I need to input the start value in radials, but the r_axis_inside value in degrees. For some reason the axis labels are rotated in my plot as well.

See code below:

set.seed(42)
df <- data.frame(x = runif(100, max = 360), y = rnorm(100))

p <- ggplot(df, aes(x, y)) +
  geom_point() +
  scale_x_continuous(limits = c(0, 360), breaks = seq(0, 360, by = 90)) +
  theme(axis.line = element_line())

p + coord_radial(theta = "x",
                 direction = -1,
                 start = -90 * pi / 180, 
                 clip = "off",
                 r_axis_inside = 135,
                 inner.radius = 0.075,
                 expand = FALSE)

Thanks for raising these points!

Also, I need to input the start value in radials, but the r_axis_inside value in degrees

The r_axis_inside value is in scale-units. If your scale indicates degrees, then r_axis_inside indicates degrees. If your scale indicates kilograms, then r_axis_inside indicates kilograms. I get how this might be confusing, but it is similar to how xlim and ylim work.

For some reason the axis labels are rotated in my plot as well.

Yeah that shouldn't happen, I'll try to figure out what is going wrong here.

Now correctly places axis at 135 degrees:

set.seed(42)
devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

df <- data.frame(x = runif(100, max = 360), y = rnorm(100))

p <- ggplot(df, aes(x, y)) +
  geom_point() +
  scale_x_continuous(limits = c(0, 360), breaks = seq(0, 360, by = 90)) +
  theme(axis.line = element_line())

p + coord_radial(theta = "x",
                 direction = -1,
                 start = -90 * pi / 180, 
                 clip = "off",
                 r.axis.inside = 135,
                 inner.radius = 0.075,
                 expand = FALSE)

^{Created on 2024-06-27 with reprex v2.1.0}