[R] axis.POSIXct: Datetime data and plotting

[Don MacQueen]

At 2:10 PM -0400 5/27/04, Barrowman, Nick wrote:
>But is there any easy way to avoid this problem?  It is confusing 
>and inconvenient that axis.POSIXct(1,d,format=f) and 
>axis.POSIXct(3,at=d,format=f) give seemingly contradictory scales.

In one case you provide the locations of the tick marks, in the other 
case you let the function decide the locations. Why, in general, 
would anyone expect the two to be the same? (And that applies 
regardless of whether we're dealing with datetime values; the regular 
axis() function doesn't always give me tick marks at locations that I 
prefer.)

Gabor Grothendieck's suggestion to use the Date class instead of 
POSIXt is a good one.

-Don

At 2:10 PM -0400 5/27/04, Barrowman, Nick wrote:
>I've run into a problem with the datetime axis generated by 
>axis.POSIXct.  It appears a similar issue was discussed in October 
>2003 under the subject line "datetime data and plotting" (see 
>https://stat.ethz.ch/pipermail/r-help/2003-October/039071.html), but 
>I wasn't able to determine whether there is a straightforward 
>solution.
>
>The code below produces a graph with apparently contradictory date 
>labels on the top and bottom axes:
>
>f <- "%Y/%m/%d"
>d <- as.POSIXct(strptime(c("2003/1/15","2003/1/16",
>   "2003/1/17","2003/1/18","2003/1/19"),format=f))
>plot(d,1:length(d),axes=F);box()
>axis(2)
>abline(v=d[3])
>axis.POSIXct(1,d,format=f)
>axis.POSIXct(3,at=d,format=f)
>
>On my copy of R (I have appended the version information below my 
>signature), this produces a graph with a vertical line that lines up 
>with "2003/01/17" on the top axis, but not on the bottom axis.
>
>The issue seems to relate to time zones: here's what d looks like on 
>my computer:
>
>>  d
>[1] "2003-01-15 Eastern Standard Time"
>[2] "2003-01-16 Eastern Standard Time"
>[3] "2003-01-17 Eastern Standard Time"
>[4] "2003-01-18 Eastern Standard Time"
>[5] "2003-01-19 Eastern Standard Time"
>
>(Actually I'm on Eastern Daylight Time, but let's ignore that for now.)
>
>Changing the axes to display not only the date but also the time 
>shows what's going on:
>
>f <- "%Y/%m/%d"
>f2 <- "%Y-%m-%d %H:%M"
>d <- as.POSIXct(strptime(c("2003/1/15","2003/1/16",
>   "2003/1/17","2003/1/18","2003/1/19"),format=f))
>plot(d,1:length(d),axes=F);box()
>axis(2)
>abline(v=d[3])
>axis.POSIXct(1,d,format=f2)
>axis.POSIXct(3,at=d,format=f2)
>
>The bottom axis put the ticks at 19:00 on each date.  This seems an 
>odd time until you note that Eastern Standard Time is 5 hours behind 
>UTC, so 19:00 EST is 00:00 UTC.
>
>But is there any easy way to avoid this problem?  It is confusing 
>and inconvenient that axis.POSIXct(1,d,format=f) and 
>axis.POSIXct(3,at=d,format=f) give seemingly contradictory scales.
>
>Many thanks for any help!
>
>Nick Barrowman, Ph.D.
>Chief Biostatistician, Chalmers Research Group
>Children's Hospital of Eastern Ontario Research Institute
>401 Smyth Road, Ottawa, Ontario, K1H 8L1, Canada
>Tel   (613) 737-7600 ext. 3971
>Fax   (613) 738-4800
>Email nbarrowman at cheo.on.ca
>URL   www.chalmersresearch.com
>
>
>>  version
>          _             
>platform i386-pc-mingw32
>arch     i386          
>os       mingw32       
>system   i386, mingw32 
>status                 
>major    1             
>minor    9.0           
>year     2004          
>month    04            
>day      12            
>language R
>
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-- 
--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA