grid.text("underlined text", y=.5, just="bottom")
w <- stringWidth("underlined text")
grid.segments(unit(.5, "npc") - 0.5*w, 
              unit(.5, "npc") - unit(1, "mm"),
              unit(.5, "npc") + 0.5*w, 
              unit(.5, "npc") - unit(1, "mm"))


grid.rect(gp=gpar(col="grey"))
grid.text("underlined text", y=.5, just="bottom")
w <- stringWidth("underlined text")
grid.segments(unit(.5, "npc") - 0.5*w, 
              unit(.5, "npc") - unit(1, "mm"),
              unit(.5, "npc") + 0.5*w, 
              unit(.5, "npc") - unit(1, "mm"))