How to rotate text to match slope of line

[dromano]

Suppose I would like to add text at the point (2, 2) so that it aligns with the line y = x.

If I use the angle parameter for geom_richtext() from the ggtext package, I can see what happens if I set it to 45^\circ:

library(tidyverse)
library(ggtext)

tibble(x = 1:3, y = 1:3) %>% 
  ggplot(aes(x, y)) +
  geom_line() +
  geom_richtext(
    x = 2, y = 2,
    label = 'slope',
    angle = 45
  )

^{Created on 2021-03-24 by the reprex package (v0.3.0)}

Not great, but even worse if I change the y-axis scale:

tibble(x = 1:3, y = 1:3) %>% 
  ggplot(aes(x, y)) +
  geom_line() +
  geom_richtext(
    x = 2, y = 2,
    label = 'slope',
    angle = 45
  ) +
  coord_cartesian(
    ylim = c(0, 4)
  )

So here's my question:

How can I represent the slope of a given line segment so that it's always aligned, no matter how the axis scale limits are chosen?

[dromano]

It seems that both the aspect ratio of the plot (set by the size of the plot pane in RStudio) and the corresponding ratio of the axis ranges are required:

library(tidyverse)
library(ggtext)

tibble(x = 1:3, y = 1:3) %>% 
  ggplot(aes(x, y)) +
  geom_line() +
  geom_richtext(
    x = 2, y = 2,
    label = 'slope',
    angle = 
      atan(
        # slope
        1 *
          # aspect ratio of plot:
          unit(0, 'npc') %>% grid::convertY('native', valueOnly = T) /
          unit(1, 'npc') %>% grid::convertX('native', valueOnly = T) /
          # ratio of y-range to x-range of plot:
          ( 2 / 2 )
      ) * 180 / pi
  )

tibble(x = 1:3, y = 1:3) %>% 
  ggplot(aes(x, y)) +
  geom_line() +
  geom_richtext(
    x = 2, y = 2,
    label = 'slope',
    angle = 
      atan(
        # slope
        1 *
          # aspect ratio of plot:
          unit(0, 'npc') %>% grid::convertY('native', valueOnly = T) /
          unit(1, 'npc') %>% grid::convertX('native', valueOnly = T) /
          # ratio of y-range to x-range of plot:
          ( 4 / 2 )
      ) * 180 / pi
  )  +
  coord_cartesian(
    ylim = c(0, 4)
  )

^{Created on 2021-03-24 by the reprex package (v0.3.0)}