I would like to add two horizontal lines representing acceptible drug levels to a trellis plot.
I tried using abline and I get an error that "plot.new has not been called."  See below.  
 
xyplot(FK~WEEK|Event1/MRN, data=FKdat.o1)
abline(h=5)
abline(h=10)
Error in int_abline(a = a, b = b, h = h, v = v, untf = untf, ...) : 
  plot.new has not been called yet

Any help that can be provided on how to do this would be appreciated.
 
Best,
Suzette 
 
Suzette Blanchard, Ph.D.
Assistant Professor, Dept. of Biostatistics
City of Hope
1500 East Duarte Rd
Duarte, CA 91010-3000
ph: (626) 256-4673 ext:64446
sblanchard@coh.org
 


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