[R] Ticks on barplots

[Michael H. Prager]

Dear Gurus,

Using R 2.4.1 on Windows XP

I am generating stacked barplots of age-composition of fish populations 
(Y) over time (X).  As there are many years, not every bars is labeled.  
When looking at the plot, it becomes difficult to associate labels with 
their bars.

We have improved this a bit by using axis() to add a tickmark below each 
bar.  Can anyone suggest a way to draw ticks ONLY at bars where a tick 
label is drawn?  Or to make such ticks longer than those where there is 
no label?

This is going into a function, so I'm hoping for a method that doesn't 
require looking at the plot first.

I have attached a PDF.

# sample code (simplified) #
mp <- barplot(t(N.age), xlab = "Year", axisnames = FALSE)
axis(side = 1, at = mp, labels = rownames(N.age), tcl = -0.75)

Thanks!

Mike Prager
NOAA, Beaufort, NC