library(lattice)
# month is defined as a factor with 12 levels, of which only levels 4 to 12
# are present in the data. 
# Note that month.abb is a predefined constant in R.
# Here it provides the labels for all 12 levels. 
df <- expand.grid(site=c("CrCr","NFSC","MFSC","SFSC","LMO","MCE","MUE","MLE"),
                  month=factor(4:12,levels=1:12,
                               labels=month.abb, ordered=TRUE))
df$conc <- rnorm(dim(df)[1])
# introduce some missing values
i <- sample(1:dim(df)[1],6)
df$conc[i] <- NA
# so there some missing points in the plot
dotplot(month~conc|site,data=df)