[R] Duration labels on plot axes

[Duncan Murdoch]

I have a variable containing a measurement of a duration in seconds 
which I would like to use in a plot, with the axes labelled in a format 
like %H:%M:%S (or possibly %Hh%Mm) if the duration is more than an hour, 
or just %M:%S if more than a minute, or just decimal seconds if short 
enough.  Is there a class with an axis.* method defined that has 
behaviour something like this?

Duncan Murdoch