我认为这就是你要找的东西:
这里的关键是axis线.该at指示哪个标记处放置标签和label说什么来调用核对符号.  
x <- sample(1:100, 10, replace = T) # just 10 random numbers
y <- sample(1:100, 10, replace = T) # 10 more random numbers
par(mar = c(10, 5, 5, 5)) 
    # increasing the 1st number to 10 makes 10 lines below axis 1
plot(x~y) # normal plot
axis(1, at = c(20, 40, 60, 80), labels = c("1", "2", "3", "4"), line = 5, col = 4) 
    # the "line" indicates which of the 10 lines made above to put the axis on