Apparently it is left to right and bottom to top, which means it is much easier to read if you set rows=1
:
# determine how many levels in var3
num <- length(unique(dtb$var3))
# plot in one row using all levels of var3
coplot(var1 ~ var2 | var3, data=dtb,
number=num,
overlap=.1,
col=rainbow(num),
type="o", # plot symbols and lines
cex=2, # make symbols larger
pch=as.character(c(seq(from=1,to=(num-1)),"T")), # use chars as symbols
rows=1) # as.character not required
# due to "T"
# plot in two rows
coplot(var1 ~ var2 | var3, data=dtb,
number=num,
overlap=.1,
col=rainbow(num),
type="o", # plot symbols and lines
cex=2, # make symbols larger
pch=as.character(c(seq(from=1,to=(num-1)),"T")), # use chars as symbols
rows=2) # as.character not required
# due to "T"