dfr <- read.table("mlijobs.txt")
dfr <- dfr[,c(2,4)]

n.checked.out <- cumsum(ifelse(dfr$V2=="OUT", 1, -1))
times <- strptime(dfr$V4, "%Y/%m/%d_%H:%M:%S")
most.checked.out <- max(n.checked.out)
when.most.checked.out <- times[which(n.checked.out==most.checked.out)]

plot(times, n.checked.out, type="s")