You want to use relative times on an axis.

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)