[R] Controlling plot axis for dates

[David Parkhurst]

I’d like to control the number of tick marks on the “x” axis of a plot, 
when the variable there is dates.  I thought to use the xaxp parameter, 
but the documentation for par says “It [i.e., xaxp] is only relevant to 
default numeric axis systems, and not for example to dates.”  My 
question, then, is how to control the number of ticks on an axis that 
represents dates?