[R] lattice and dates

[Michael A. Miller]

I've got some data that is recorded at various times during the
day.    I create a time stamp is created from date and time
strings like this:
  
  > timestamp <- strptime( paste( datestring, timestring), "%D %X" )

Then I'd like to use lattice to plot the time dependence by doing
something like 

  > xyplot( value ~ timestamp | subject )

This fails with 

  Error in as.double.default(x) : (list) object cannot be coerced to vector type 14
  In addition: Warning message: 
  Both x and y should be numeric in: xyplot( value ~ timestamp | subject )

I was sure I saw something like this on r-help recently, but I
can't seem to track it down now.  Can anyone help me out?

Thanks, Mike

-- 
Michael A. Miller                               mmiller3 at iupui.edu
  Imaging Sciences, Department of Radiology, IU School of Medicine
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