ligne de tracé

#h - horizontal line 
abline(h = 0.5, lty=4, col="grey")

#v - vertical line
abline(v = c(10, 20), lty = 2) #two lines at 10 and 20

#lty - line type
0, 1, 2, 3, 4, 5, 6
“blank”, “solid”, “dashed”, “dotted”, “dotdash”, “longdash”, “twodash”
Diana H