R help - Jul 2007 - duplicate DATE at in lattice scale handled differently from base graphics OR lattice numeric scales

When at, label chains contain duplicate at values, axis ticks are  
dropped.  I believe this is handled incorrectly for Date ats in  
lattice 0.15-4 when compared to how it is handled for numeric, or for  
dates in base plot.  This can result in mis-labelled axes in some  
circumstances.  I have included executable examples.

R: 2.5.0

numeric base plot:

plot(1:10, axes=F); axis(1, at=c(2,4,4,6), labels=letters[1:4])

shows labels a, b, d

date scale in base plot:

plot((as.Date("2007-01-01") + 1:10), 1:10, axes=F); axis(1, at= 
(as.Date("2007-01-01")+c(2,4,4,6)), labels=letters[1:4])

shows labels a, b, d

numeric scale in lattice plot

xyplot(1:10 ~ 1:10, scales=list(x=list(relation="free", at=c 
(2,4,4,6), labels=list(letters[1:4]))))

shows labels a, b, d

date scale in lattice plot

xyplot(1:10 ~ (as.Date("2007-01-01") + 1:10) , scales=list(x=list 
(relation="free", at=list(as.Date("2007-01-01")+c(2,4,4,6)),
labels=list(letters[1:4]))))

shows labels a, b, c

Since this results in label c being placed at position 6, not 4, I  
feel this is a bug.

-Alex Brown

On 7/30/07, Alex Brown <alex at transitive.com>
wrote:
> When at, label chains contain duplicate at values, axis ticks are
> dropped.  I believe this is handled incorrectly for Date ats in
> lattice 0.15-4 when compared to how it is handled for numeric, or for
> dates in base plot.  This can result in mis-labelled axes in some
> circumstances.  I have included executable examples.
>
> R: 2.5.0
>
> numeric base plot:
>
> plot(1:10, axes=F); axis(1, at=c(2,4,4,6), labels=letters[1:4])
>
> shows labels a, b, d
>
> date scale in base plot:
>
> plot((as.Date("2007-01-01") + 1:10), 1:10, axes=F); axis(1,
at> (as.Date("2007-01-01")+c(2,4,4,6)), labels=letters[1:4])
>
> shows labels a, b, d
However,
> foo = as.Date("2007-01-01")
> plot(foo + 1:10, 1:10, axes = FALSE)
> axis.Date(1, at = foo + c(2,4,4,6), labels = letters[1:4])
Error in axis(side, at = z, labels = labels, ...) :
  'at' and 'labels' lengths differ, 3 != 4

And this code is used almost unchanged in lattice, leading to
> str(lattice:::formattedTicksAndLabels.Date(range(foo + 1:10), at = foo +
c(2, 4, 4, 6),  labels = letters[1:4]))
List of 4
 $ at           : num [1:3] 13516 13518 13520
 $ labels       : chr [1:4] "a" "b" "c"
"d"
 $ check.overlap: logi FALSE
 $ num.limit    : num [1:2] 13515 13524

That is, 'at' is now of length 3 (and unlike axis.Date, this does not
subsequently cause an error, which probably would have been
preferable).

I will add a fix.

-Deepayan

> numeric scale in lattice plot
>
> xyplot(1:10 ~ 1:10, scales=list(x=list(relation="free", at=c
> (2,4,4,6), labels=list(letters[1:4]))))
>
> shows labels a, b, d
>
> date scale in lattice plot
>
> xyplot(1:10 ~ (as.Date("2007-01-01") + 1:10) , scales=list(x=list
> (relation="free",
at=list(as.Date("2007-01-01")+c(2,4,4,6)),
> labels=list(letters[1:4]))))
>
> shows labels a, b, c
>
> Since this results in label c being placed at position 6, not 4, I
> feel this is a bug.
>
> -Alex Brown