How to add minor tick marks to an axis besides its main tick marks?

Question

I have two questions.

1. I was wondering how I can add two words, one appearing in place of "0", the other appearing in place of "10" on the X and Y axes while preserving the rest of the remaining numbers on the axes (i.e., 1 thru 9)?
2. How can I add minor tick marks to both axes?

Here is my R code, with no success:

plot(0:10,0:10,type="n",axes=FALSE,xlab="JBL",ylab="PRAC")

axis(side=1,at=c("AL",1,2,3,4,5,6,7,8,9,"BL"),las=1)
axis(side=2,at=c("CL",1,2,3,4,5,6,7,8,9,"DL"),las=1)
box()

Answer 1

For the first question, you should use labels not at. Read ?axis.

For the second, you can call axis or Axis twice on the same axis, but using rug is easier. It is a wrapper function for Axis. For example, you can do:

plot(0:10, 0:10, axes = FALSE)  ## `axes` is an argument to `plot.default`
axis(1, at = 0:10, labels = c("AL",1,2,3,4,5,6,7,8,9,"BL"))
rug(x = 0:9 + 0.5, ticksize = -0.01, side = 1)
axis(2, at = 0:10, labels = c("CL",1,2,3,4,5,6,7,8,9,"DL"))
rug(x = 0:9 + 0.5, ticksize = -0.01, side = 2)

You can see how rug works in its source code:

Axis(side = side, at = x, labels = FALSE, lwd = 0, lwd.ticks = lwd, 
    col.ticks = col, tck = ticksize, ...)

Read ?par for how tck works. The default tck for an axis is -0.02, so if you want a smaller one for "minor axis", set it to half. Here, the sign means direction. Positive values give ticks pointing inside the plot, while negative ones give ticks pointing outside the plot.

Answer 2

For the minor ticks, you could use the minor.tick function from the hmisc package.

minor.tick(nx=2, ny=2, tick.ratio=0.5, x.args = list(), y.args = list())

https://www.rdocumentation.org/packages/Hmisc/versions/4.1-1/topics/minor.tick