Question

how can I have a data set of only time intervals (no dates) in R, like the following:

TREATMENT_A TREATMENT_B

1:01:12 0:05:00

0:34:56 1:08:09

and compute mean times, etc, and draw boxplots with time intervals in the y-axis?

I am new to R, and I searched for this but found no example in the net.

Thanks

Answer 1

The chron-package has a ‘times’ class that supports arithmetic. You could also do all of that with POSIXct objects and format the date-time output to not include the date. I thought axis.POSIXct function has a format argument that should let you have time outputs. However, it does not seem to get dispatched properly, so I needed to construct the axis “by hand.”

dft <- data.frame(x= factor( sample(1:2, 100, repl=TRUE)), 
                  y= Sys.time()+rnorm(100)*4000 )
boxplot(y~x, data=dft, yaxt='n')
axis(2, at=seq(from=range(dft$y)[1], to =range(dft$y)[2], by=3000) , 
         labels=format.POSIXct(seq(from=range(dft$y)[1], to =range(dft$y)[2], by=3000), 
                                 format ="%H:%M:%S") )

There did turn out to be an appropriate method, Axis.POSIXt (to which I thought boxplot should have been turning for plotting, but it did not seem to recognize the class of the ‘y’ argument):

boxplot(y~x, data=dft, yaxt='n')
Axis(side=2, x=range(dft$y), format ="%H:%M:%S")

Regarding your request for something “simpler”, take a look at theis ggplot2 based solution, using the dft dataframe defined above with POSIXct times. (I did try with the chron-times object but got a message saying ggplot did not support that class):

require(ggplot2); p <- ggplot(dft, aes(x,y))
p + geom_boxplot()