alpha in geom_segment not working [duplicate]

Question

I was trying few refinements in my plot and encounter that alpha in geom_segment is not working properly. For minimum working example check this:

ggplot(mtcars, aes(hp, mpg)) + 
  geom_point() + 
  geom_segment(aes(x = 100, xend = 200, y = 20, yend = 20), 
  inherit.aes = FALSE, 
  size = 10, 
  alpha = 0.5, 
  color = "blue")

However, if you change the alpha to really low value such as 0.005, 0.001 appears working. You can only see some effect from 0.05 to 0.001.

Aren't the alpha values supposed to change in a linear manner between 0 and 1 or have I understood incorrectly?

Answer 1

ggplot2 is drawing many segments, one on top of each other making the segment opaque. You can solve it by removing the data from the ggplot function and add it to the required layers. Similar problem with other geoms here and here.

ggplot() + 
    geom_point(data=mtcars, aes(hp, mpg)) + 
    geom_segment(aes(x = 100, xend = 200, y = 20, yend = 20), 
                 inherit.aes = FALSE, 
                 size = 10, 
                 alpha = 0.5, 
                 color = "blue")

Another option is to use annotate as Eric did:

ggplot(mtcars) +
    geom_point(aes(hp, mpg)) +
    annotate(
      'segment',
      x = 100,
      xend = 200,
      y = 20,
      yend = 20,
      size = 10,
      colour = "blue",
      alpha = 0.5
    )