Plotting times at night and getting plot limits correct

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

Biodiversity and Climate Research Centre
Senckenberganlage 25
D-60325 Frankfurt am Main,
Germany

Tel: +49 69 798 40226
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Peter Dalgaard, Professor,
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Plotting times at night and getting plot limits correct

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