[R] Inconsistent lattice scales$x$at,label behaviour for POSIXct

[Alex Brown]

I have encountered the following behaviour in lattice in 2.6.1 (and  
2.4.0) which differs depending upon the type you use.  I believe the  
numeric behaviour to be correct, and the POSIXct behaviour to be in  
error.

When the x data and x axis in a lattice graph are POSIXct, and when  
using scales$x$at and scales$x$labels to add custom labels:

If the first visible at value is not the first specified at value (due  
to x limit settings), the first visible label nonetheless receives the  
first specified label, instead of the label corresponding to the first  
visible at.

In the following example, at = baseval + 1:4, label = letters[1:4]

I have set it up so that baseval + 1:2 are not visible in the graph

for numeric and Date types, the visible labels are letters[3:4] - ie  
"c" "d"

for POSIXct, the visible labels are letters[1:3] - "a" "b".


Simplest to show this by example:

# numeric

baseval=0;
xyplot(1:10 ~ (baseval + 1:10) , scales=list(x=list
(at=baseval+1:4, labels=letters[1:4])),xlim=baseval+c(3,10))

# Date

baseval = as.Date("2007-01-01");
xyplot(1:10 ~ (baseval + 1:10) , scales=list(x=list
(at=baseval+1:4, labels=letters[1:4])),xlim=baseval+c(3,10))

# POSIXct

baseval = as.POSIXct("2007-01-01");
xyplot(1:10 ~ (baseval + 1:10) , scales=list(x=list
(at=baseval+1:4, labels=letters[1:4])),xlim=baseval+c(3,10))

in particular, compare the Date and POSIXct versions

-Alex Brown