A horribly pedestrian alternative to plotrix:

 

barplot(x,beside=TRUE,  ylim=c(0,90),  etc.)

par('usr')    # just getting plot limits

[1]  0.44  15.56  0.00  90.00

par(new=T)

lines(c(0.44,15.56),c(20,20))

lines(c(0.44,15.56),c(40,40))

etc.


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