Here is the code I used so far:
ord_interleave_elements <- order(c(seq_along(chemo_dataset$blast_percent), seq_along(chemo_dataset$d15_absol), seq_along(chemo_dataset$d29_absol)))
barplot(unlist(c(chemo_dataset$blast_percent, chemo_dataset$d15_absol, chemo_dataset$d29_absol))[ord_interleave_elements], beside = TRUE, col = c("black", "grey", "lightgrey"), ylab = "Leukemic cell (%) in bone marrow", xlab = "Case number", family = windowsFont("times"))
How can I position the patient ID for each group of 3?