[R] datetime data and plotting

[Jacob Etches]

If I take the following simple data:

YEAR MONTH DAY WEIGHT.KG
2003 10 6 1.2
2003 10 12 1.2
2003 10 16 1.3

and format the date data and plot it:

dates <- strptime(paste(DAY,MONTH,YEAR),"%d%m%Y")
plot(c(min(dates),max(dates)),c(0,max(WEIGHT.KG)),
 xlab="Date",ylab="Weight (kg)",type="n")
 lines(dates,WEIGHT.KG)
 points(dates,WEIGHT.KG)

I find that the data points are all plotted at (x-1,y),
where x is in days.  Have I requested this behaviour 
accidentally?  I'm using R-1.8 on OS X.

Printing the dates object looks correct, and simple 
manipulations such as max(dates)-min(dates) behave 
normally.

Jacob Etches

Doctoral candidate
Dept of Public Health Sciences
University of Toronto