ggplot2: Plotting Dates, Hours and Minutes

February 24, 2010
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Plotting timeseries with dates on x-axis and times on y-axis can be a bit tricky in ggplot2. However, with a little trick this problem can be easily overcome.

Let’s assume that I wanted to plot when the sun rises in London in 2010.

sunriset function in maptools package calculates the sunrise times using algorithms provided by the National Oceanic & Atmospheric Administration (NOAA). I need to input the location details in a matrix format and specify for which dates I require the sunrise times.

> library(maptools)
> x_seq <- seq(from = as.POSIXct("2010-01-01", tz = "GMT"),
+     length.out = 365, by = "days")
> coord <- matrix(c(-0.13, 51.5), nrow = 1)
> sunrise <- sunriset(coord, x_seq, direction = "sunrise",
+     POSIXct.out = TRUE)
> head(sunrise, 3)
   day_frac                time
1 0.3376285 2010-01-01 08:06:11
2 0.3375441 2010-01-02 08:06:03
3 0.3374209 2010-01-03 08:05:53

The time variable now includes information about both the date and time of sunrise in class POSIXct.

I would like to plot date on x-axis and time on y-axis, thus the time element needs to be extracted first. However, as the times must be in POSIXct (only times of class POSIXct are supported in ggplot2), a two-step conversion is needed.

First the time is converted to a character vector, effectively stripping all the date information. The time is then converted back to POSIXct with today’s date – the date is of no interest to us, only the hours-minutes-seconds are.

> sunrise$hms <- format(sunrise$time, format = "%H:%M:%S")
> sunrise$hms <- as.POSIXct(sunrise$hms, format = "%H:%M:%S")
> head(sunrise, 3)
   day_frac                time                 hms
1 0.3376285 2010-01-01 08:06:11 2010-01-20 08:06:11
2 0.3375441 2010-01-02 08:06:03 2010-01-20 08:06:03
3 0.3374209 2010-01-03 08:05:53 2010-01-20 08:05:53

Now everything is set for plotting.

> library(ggplot2)
> ggplot(sunrise, aes(time, hms)) + geom_line()
hours_minutes_sunrise-008.png

The default x-axis labels could be made somewhat clearer:

> last_plot() + scale_x_datetime("", format = "%b") +
+     ylab("")
hours_minutes_sunrise-010.png

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