Placing the text appropriately in ggplot

This is based on my previous post in Adding shapes near to the text in ggplot

Based on the accepted answer from @Tim G

library(ggplot2)
library(ggalt)
library(ggpp)
library(grid)

df <- data.frame(trt=LETTERS[1:5], l=c(20, 40, 10, 30, 50), r=c(70, 50, 30, 60, 80))

ggplot(df, aes(y = trt, x = l, xend = r)) +
  geom_dumbbell(size = 3, color = "#e3e2e1",
                colour_x = "#5b8124", colour_xend = "#bad744") +
  geom_text_npc(aes(npcx = 'center', npcy = 'bottom'), label = expression('No. of'~italic('AA')~'isolates with corresponding types'), parse = TRUE) +
  
  annotation_custom(
    grob = rectGrob(
      x = unit(0.5, "npc"), y = unit(0.04, "npc"),
      width = unit(0.4, "npc"), height = unit(0.011, "npc"),
      gp = gpar(fill = "gray60", col = NA)
    )
  ) 

I wonder if it possible to push the expression generated by geom_text_npc() (along with the underline created) further below just little above the bottom edge of the plot, however without modifying the y-axis. I tried with passing nudge_y = -1 in geom_text_npc(), but it didnt work.

Thanks for your time and attention.

You can give npcy as exact number in your geom_text_npc(aes(npcx = 'center', npcy = 0.02) and in annotation_custom adjust y = unit(0.04, "npc") to y = unit(0.01, "npc").

As pointed out correctly by @Edward, if you don't want the warning, pass a data argument to geom_text_npc() containing only 1 row (since you have only one label), which is basically annotation since the text label is not connected to your dumbell data.

library(ggplot2)
library(ggalt)
library(ggpp)
library(grid)

df <- data.frame(trt=LETTERS[1:5], l=c(20, 40, 10, 30, 50), r=c(70, 50, 30, 60, 80))

ggplot(df, aes(y = trt, x = l, xend = r)) +
  geom_dumbbell(size = 3, color = "#e3e2e1",
                colour_x = "#5b8124", colour_xend = "#bad744") +
  geom_text_npc(data=data.frame(), # pass empty data here
     aes(npcx = 'center', 
         npcy = 0.02), # adjust y-pos of text here
         label = expression('No. of'~italic('AA')~'isolates with corresponding types'
     ), parse = TRUE) +
  
  annotation_custom(
    grob = rectGrob(
      x = unit(0.5, "npc"), y = unit(0.01, "npc"), # adjust y-pos of the grey bar
      width = unit(0.4, "npc"), height = unit(0.011, "npc"),
      gp = gpar(fill = "gray60", col = NA)
    )
  ) 

Edit While this works, Stefan's answer provides a cleaner / easier to adjust approach and I woould recommend it.