8.18 Using Relative Times on an Axis

8.18.1 Problem

You want to use relative times on an axis.

8.18.2 Solution

Times are commonly stored as numbers. For example, the time of day can be stored as a number representing the hour. Time can also be stored as a number representing the number of minutes or seconds from some starting time. In these cases, you map a value to the x- or y-axis and use a formatter to generate the appropriate axis labels (Figure 8.40):

# Convert WWWusage time-series object to data frame
www <- data.frame(
  minute = as.numeric(time(WWWusage)),
  users  = as.numeric(WWWusage)
)

# Define a formatter function - converts time in minutes to a string
timeHM_formatter <- function(x) {
  h <- floor(x/60)
  m <- floor(x %% 60)
  lab <- sprintf("%d:%02d", h, m) # Format the strings as HH:MM
  return(lab)
}

# Default x axis
ggplot(www, aes(x = minute, y = users)) +
  geom_line()

# With formatted times
ggplot(www, aes(x = minute, y = users)) +
  geom_line() +
  scale_x_continuous(
    name = "time",
    breaks = seq(0, 100, by = 10),
    labels = timeHM_formatter
  )

Figure 8.40: Top: relative times on x-axis; bottom: with formatted times

8.18.3 Discussion

In some cases it might be simpler to specify the breaks and labels manually, with something like this:

scale_x_continuous(
  breaks = c(0, 20, 40, 60, 80, 100),
  labels = c("0:00", "0:20", "0:40", "1:00", "1:20", "1:40")
)

In the preceding example, we used the timeHM_formatter() function to convert the numeric time (in minutes) to a string like "1:10":

timeHM_formatter(c(0, 50, 51, 59, 60, 130, 604))
#> [1] "0:00"  "0:50"  "0:51"  "0:59"  "1:00"  "2:10"  "10:04"

To convert to HH:MM:SS format, you can use the following formatter function:

timeHMS_formatter <- function(x) {
  h <- floor(x/3600)
  m <- floor((x/60) %% 60)
  s <- round(x %% 60)                       # Round to nearest second
  lab <- sprintf("%02d:%02d:%02d", h, m, s) # Format the strings as HH:MM:SS
  lab <- sub("^00:", "", lab)               # Remove leading 00: if present
  lab <- sub("^0", "", lab)                 # Remove leading 0 if present
  return(lab)
}

Running it on some sample numbers yields:

timeHMS_formatter(c(20, 3000, 3075, 3559.2, 3600, 3606, 7813.8))
#> [1] "0:20"    "50:00"   "51:15"   "59:19"   "1:00:00" "1:00:06" "2:10:14"

8.18.4 See Also

See Recipe 15.21 for information about converting time series objects to data frames.