annotation_custom with npc coordinates in ggplot2

Question

I would like to draw a box at the bottom of each panel, where each panel may have different y-ranges. The box should always be from 3 to 7 on the x-axis and from t

Answer 1

You could do,

g <- rectGrob(y=0,height = unit(0.05, "npc"), vjust=0, 
              gp=gpar(fill="black"))
p + annotation_custom(g, xmin=3, xmax=7, ymin=-Inf, ymax=Inf)