[R] matplot with dates/times on horizontal axis

[Alex van der Spek]

 >matplot(timestamp,xymatrix,type='l')

where timestamp is a vector filled with POSIXct objects and xymatrix is 
a numeric 2x2 matrix plots but the horizontal axis labels are raw 
unformatted timestamps.

I would like to format these in any of the available codes for strftime, 
for instance format="%H:%M".

Passing a vector of formatted strings does not work. Any obvious other 
ways fail upon me as well.

Any ideas to make this work?

Thanks in advance,
Alex van der Spek