How can I add annotations below the x axis in ggplot2?

This can be done using annotation_custom(). I am drawing from this answer.

The difficulty is that ggplot clips annotations that are placed outside the plot area, which is what you want to do. But clipping can be turned off.

annotation_custom() uses grobs, so you first need to create the them:

library(grid)
text_high <- textGrob("Highest\nvalue", gp=gpar(fontsize=13, fontface="bold"))
text_low <- textGrob("Lowest\nvalue", gp=gpar(fontsize=13, fontface="bold"))

Next, you set up the plot and store it:

p <-
df1 %>% ggplot(aes(x , y )) + geom_bar(stat = "identity") +
   scale_y_continuous(labels = percent) +
   theme(plot.margin = unit(c(1,1,2,1), "lines")) +
   annotation_custom(text_high,xmin=1,xmax=1,ymin=-0.07,ymax=-0.07) + 
   annotation_custom(text_low,xmin=5,xmax=5,ymin=-0.07,ymax=-0.07)

The third line makes sure that there is enough room beneath the plot for your labels and the last two add the annotations. The position is given as min and max values for both coordinates. The grob will be centered in the region that is defined by these coordinates. In the present situation, it seems easiest to simply define a point by setting min and max values identical.

The last step is to turn off clipping such that objects outside the plot area (i.e., the annotations) are also drawn. For ggplot2 3.0.0 and newer this can be done using coord_cartesian() (see the answer by tfad334):

p + coord_cartesian(clip = "off")

With older versions of ggplot2 the procedure is slightly more complicated:

gt <- ggplot_gtable(ggplot_build(p))
gt$layout$clip[gt$layout$name == "panel"] <- "off"
grid.draw(gt)

The last line draws the plot.

With ggplot2 version 3.0.0, you won't need gtable to turn clipping off in Stibu's answer. Use coord_cartesian() to achieve the same thing:

library(gridExtra)
df1 %>% ggplot(aes(x , y )) + geom_bar(stat = "identity")+
scale_y_continuous(labels = percent)+
theme(plot.margin = unit(c(1,1,2,1), "lines")) +
annotation_custom(text_high,xmin=1,xmax=1,ymin=-0.07,ymax=-0.07) + 
annotation_custom(text_low,xmin=5,xmax=5,ymin=-0.07,ymax=-0.07)+
coord_cartesian(ylim=c(0,0.35), clip="off")