[Rd] patch for axis.POSIXct (related to timezones)

[Prof Brian Ripley]

This is the appropriate forum, and thank you for the comments.
At a quick look it might be simpler to use the 'tz' argument to 
as.POSIXlt, but I'll look in more detail and commit a change later 
today.

On Wed, 4 Mar 2009, Dan Kelley wrote:

>
> I am finding that axis.POSIXct uses the local timezone for deciding where to
> put tic marks, even if the data being plotted are in another time zone.  The
> solution is to use attr() to copy from the 'x' (provided as an argument) to
> the 'z' (used for the 'at' locations).
>
> I have pasted my proposed solution in section 1 below (as a diff).  Then, in
> section 2, I'll put some test code that I wrote, when I was figuring this
> out.
>
> I am not entirely sure whether it's OK, or helpful, to post a diff here.  I
> don't understand the R development model well enough to know how to suggest
> changes, and ?axis.POSIXct does not list an author, so that's why I'm
> posting here.
>
> (All of this matters because I am sharing code with people working in
> different timezones; I want the timezone of the data to carry over to the
> graph.)
>
> Section 1: patch to axis.POSIXct
> ====================
>
> (Note that the line numbers may be wrong; I'm not working with the source
> for axis.POSIXct, but rather with the output from a listing of the function
> in the terminal).
>
> ~$ diff -Naur axis.POSIXct.R my.axis.POSIXct.R
> --- axis.POSIXct.R	2009-03-04 16:22:18.000000000 -0400
> +++ my.axis.POSIXct.R	2009-03-04 16:22:56.000000000 -0400
> @@ -1,4 +1,4 @@
> -axis.POSIXct <- function (side, x, at, format, labels = TRUE, ...)
> +my.axis.POSIXct <- function (side, x, at, format, labels = TRUE, ...)
> {
>     mat <- missing(at) || is.null(at)
>     if (!mat)
> @@ -9,6 +9,7 @@
>     else 3:4]
>     d <- range[2] - range[1]
>     z <- c(range, x[is.finite(x)])
> +    attr(z, "tzone") <- attr(x, "tzone")
>     if (d < 1.1 * 60) {
>         sc <- 1
>         if (missing(format))
> @@ -41,6 +42,7 @@
>         zz <- pretty(z/sc)
>         z <- zz * sc
>         class(z) <- c("POSIXt", "POSIXct")
> +        attr(z, "tzone") <- attr(x, "tzone")
>         if (sc == 60 * 60 * 24)
>             z <- as.POSIXct(round(z, "days"))
>         if (missing(format))
> @@ -48,6 +50,7 @@
>     }
>     else if (d < 1.1 * 60 * 60 * 24 * 365) {
>         class(z) <- c("POSIXt", "POSIXct")
> +        attr(z, "tzone") <- attr(x, "tzone")
>         zz <- as.POSIXlt(z)
>         zz$mday <- zz$wday <- zz$yday <- 1
>         zz$isdst <- -1
> @@ -65,6 +68,7 @@
>     }
>     else {
>         class(z) <- c("POSIXt", "POSIXct")
> +        attr(z, "tzone") <- attr(x, "tzone")
>         zz <- as.POSIXlt(z)
>         zz$mday <- zz$wday <- zz$yday <- 1
>         zz$isdst <- -1
> ~$
>
>
> Section 2.  Test code
> =============
>
> # fake some data, and draw a vertical line at midnight ... note how
> # the latter will be in the wrong place (unless the computer is set to UTC).
> tc <- c("2008-06-28 15:50:00 UTC","2008-06-28 20:50:00 UTC","2008-06-29
> 01:50:00 UTC",
>       "2008-06-29 06:50:00 UTC","2008-06-29 11:50:00 UTC","2008-06-29
> 16:50:00 UTC",
>       "2008-06-29 21:50:00 UTC","2008-06-30 02:50:00 UTC","2008-06-30
> 07:50:00 UTC",
>       "2008-06-30 12:50:00 UTC")
> t <- as.POSIXct(tc, tz="UTC")           # note using UTC
>
> p <- c(2.4998, 0.4687, 2.7120, 2.0676, 0.5614, 2.6121, 0.5161, 2.9572,
> 2.2567, 0.3820)
>
> t0 <- as.POSIXct("2008-06-29 00:00:00", tz="UTC")
> par(mfrow=c(2,1))
> plot(t, p, type='l')
> abline(v=t0, col='red')
>
> plot(t, p, type='l', axes=FALSE)
> box()
> axis(2)
> source("~/my.axis.POSIXct.R")
> my.axis.POSIXct(side=1, x=t)
> abline(v=t0, col='red')
>
>
>
> -- 
> View this message in context: http://www.nabble.com/patch-for-axis.POSIXct-%28related-to-timezones%29-tp22338700p22338700.html
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>
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>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595