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

[Dan Kelley]

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')



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