An answer to this closed StackOverflow question about reversing a time axis in ggplot2.
date = c("2011-11-15", "2011-11-16", "2011-11-17", "2011-11-19")
start = c("12:01:27", "12:01:25", "12:01:02", "12:01:12")
end = c("12:30:15", "12:32:15", "12:39:12", "12:30:18")
df = data.frame(date = as.POSIXct(date),
ystart = as.POSIXct(start, format="%H:%M:%S"),
yend = as.POSIXct(end, format="%H:%M:%S"),