R - ggplot2 - How to use limits on POSIX axis?

Question

What is the smartest way to manipulate POSIX for use in ggplot axis?

I am trying to create a function for plotting many graphs (One per day) spanning a period of weeks, using POSIX time for the x axis.

To do so, I create an additional integer column DF$Day with the day, that I input into the function. Then, I create a subset using that day, which I plot using ggplot2. I figured how to use scale_x_datetime to format the POSIX x axis. Basically, I have it show the hours & minutes only, omitting the date.

Here is my question: How can I set the limits for each individual graph in hours of the day?

Below is some working, reproducible code to get an idea. It creates the first day, shows it for 3 seconds & the proceeds to create the second day. But, each days limits is chosen based on the range of the time variable. How can I make the range, for instance, all day long (0h - 24h)?

DF <- data.frame(matrix(ncol = 0, nrow = 4))

DF$time <- as.POSIXct(c("2010-01-01 02:01:00", "2010-01-01 18:10:00", "2010-01-02 04:20:00", "2010-01-02 13:30:00"))
DF$observation <- c(1,2,1,2)
DF$Day <- c(1,1,2,2)

for (Individual_Day in 1:2) { 
Day_subset <- DF[DF$Day == as.integer(Individual_Day),]

print(ggplot( data=Day_subset, aes_string( x="time", y="observation") ) + geom_point() +
      scale_x_datetime( breaks=("2 hour"), minor_breaks=("1 hour"), labels=date_format("%H:%M")))

Sys.sleep(3) }

Answer 1

Well, here's one way.

# ...
for (Individual_Day in 1:2) { 
Day_subset <- DF[DF$Day == as.integer(Individual_Day),]
lower <- with(Day_subset,as.POSIXct(strftime(min(time),"%Y-%m-%d")))
upper <- with(Day_subset,as.POSIXct(strftime(as.Date(max(time))+1,"%Y-%m-%d"))-1)
limits = c(lower,upper)

print(ggplot( data=Day_subset, aes( x=time, y=observation) ) + 
        geom_point() +
        scale_x_datetime( breaks=("2 hour"), 
                          minor_breaks=("1 hour"), 
                          labels=date_format("%H:%M"),
                          limits=limits)
      )
}

The calculation for lower takes the minimum time in the subset and coerces it to character with only the date part (e.g., strips away the time part). Converting back to POSIXct generates the beginning of that day.

The calculation for upper is a little more complicated. You have to convert the maximum time to a Date value and add 1 (e.g., 1 day), then convert to character (strip off the time part), convert back to POSIXct, and subtract 1 (e.g., 1 second). This generates 23:59 on the end day.

Huge amount of work for such a small thing. I hope someone else posts a simpler way to do this...