Hmmm... is this a (unknown or even a) bug ? Not sure this is what OP was
getting at and I haven't gone through the lattice docs to see if this is
mentioned, but, with unequal sizes the extended formula is not producing
what one (or at least I) might expect:

 require(latticeExtra)
set.seed(4321) ; x <- rnorm(20) ; y <- rnorm(10 , 50)
c(dsp <- densityplot( ~ x + y) ,
   densityplot( ~ data , groups = which , make.groups(x, y) ) )

# because
 all.equal(dsp$panel.args[[1]]$x, c(x, y) )
# but regroups ignoring original lengths
summary(dsp$panel.args.common$groups)

An obvious BTW: problem persists using n={19,11} or n = {20,11}

sessionInfo()
R version 2.15.1 (2012-06-22)
Platform: x86_64-pc-linux-gnu (64-bit)
attached base packages:
[1] grDevices utils     datasets  stats     graphics  methods   base

other attached packages:
[1] nlme_3.1-104        latticeExtra_0.6-24 RColorBrewer_1.0-5
[4] lattice_0.20-10


On Wed, Feb 13, 2013 at 6:38 AM, Michael Stob <stob@calvin.edu> wrote:

> densityplot(~x+y) does what I expect it to do if x and y have equal
> length.  I know how to get what I want if x and y have different lengths.
> But what is this actually doing if x and y have different lengths?    The
> relevant example is
>
> x=rnorm(10)
> y=rnorm(50,1)
> densityplot(~x+y)
>
>
>
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