Changing background color for a text annotation to increase contrast and visibility

Try geom_label instead:

ggplot() + 
  geom_hline(yintercept = 0) + 
  labs(x = "", y = "") +
  geom_label(aes(x = 0, y = 0, label = "Here is a line"), fill = "green")

Building on this answer, but avoiding the use of geom_label() so that the label draws only once, not once for every row of plotted data (as correctly pointed out in this comment):

You can still use annotate(), which is the preferred approach for a one-off annotation, but use label instead of text as the geom.

Likewise you could supply geom="segment" to draw a line, etc...

ggplot() + 
  geom_hline(yintercept=0) + 
  annotate(geom="label",x=0,y=0,label="Here is a line", fill="green")