[R] Plotting times at night and getting plot limits correct

[peter dalgaard]

On 12 May 2015, at 16:34 , Bob O'Hara <rni.boh at gmail.com> wrote:

> I'm helping colleagues with analysis of frog calls at night,

What do you do during the daytime then? 

> and they
> want to plot call statistics against time. This means we hit a
> problem: we want the x-axis to start at (say) 18:00 and end at (say)
> 06:00. I'm reluctant to use the date as well, because we have data
> from several dates, but only want to plot against time of day.
> 
> Here's some code to illustrate the problem (don't worry about the data
> being outside the range of the plot: this is only for illustration).
> 
> library(chron)
> Times <- chron(times.=paste(c(18:23,0:9),":30:00", sep=""))
> Thing <- rnorm(length(Times)) # just something for the y-axis
> 
> plot(Times,Thing) # x-axis wrong
> plot(Times,Thing, xlim=chron(times.=c("05:00:00", "18:00:00"))) # x-axis right
> plot(Times,Thing, xlim=chron(times.=c("18:00:00", "05:00:00"))) #
> would like this to work...
> 
> Can anyone suggest a solution?

It may be sheer luck, but this seems to work OK:

> Times <- as.POSIXct("1970-01-01 18:00", tz="UTC")+as.difftime(seq(0,12,.5),units="hours")
> Things<-rnorm(25)
> plot(Times,Things) 

I think the chron route is doomed because of the fundamental confusion between going backwards in time and crossing midnight. With a little diligence you should be able to shift dates to a common origin.

-pd

> 
> Bob
> 
> -- 
> Bob O'Hara
> 
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-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Office: A 4.23
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com