please cite the following code at stats.oarc.ucla.edu

default label

x<-1:10
y<-x*x
plot(x, y, type="b")

change the x-axes label

par(xaxt="n")
plot(x, y, type="b")
lablist<-as.vector(c(1:10))
axis(1, at=seq(1, 10, by=1), labels = FALSE)
text(seq(1, 10, by=1), par("usr")[3] - 0.2, labels = lablist, srt = 45, pos = 1, xpd = TRUE)

change the x&y axes labels

par(xaxt="n",yaxt='n')
plot(x, y, type="b")
lablist.x<-as.vector(c(1:10))
lablist.y<-as.vector(c("zero", "twenty", "forty", "sixty", "eighty", "one hundred"))
axis(1, at=seq(1, 10, by=1), labels = FALSE)
text(x = seq(1, 10, by=1), par("usr")[3] - 0.2, labels = lablist.x, srt = 45, pos = 1, xpd = TRUE)
axis(2, at=seq(0, 100, by=20), labels = FALSE)
text(y = seq(0, 100, by=20), par("usr")[1], labels = lablist.y, srt = 45, pos = 2, xpd = TRUE)