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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...