What is the unit of text height in ggplot2 annotate?

Question

My goal is to adjust the hight of annotation according to coordinate scale. To do so, could I know the unit of size in annotate function? (Below, I put size = 10 and it seems the size 10 is about 0.0125 or so.)

library(ggplot2)

ggplot() + 
  geom_point(aes(1, 1)) +
  annotate(
    "text", x = 1, y = 1, label = "My label",
    size = 10,
    hjust = 0
  )

^{Created on 2020-11-10 by the reprex package (v0.3.0)}

Answer 1

The size is in mm I believe. You can specify size in points by dividing by the constant .pt, which converts points into mm. Note that font size in theme elements is specified in points and doesn't require division by .pt.

library(ggplot2)

ggplot() + 
  geom_point(aes(1, 1)) +
  annotate(
    "text", x = 1, y = 1, label = "My label",
    size = 15/.pt,
    hjust = 0
  ) +
  theme(
    axis.title = element_text(size = 15),
    axis.text = element_text(size = 15)
  )

^{Created on 2020-11-10 by the reprex package (v0.3.0)}