lattice: how to "center" a subtitle?

On Apr 4, 2011, at 9:03 PM, David Winsemius wrote:

On Apr 4, 2011, at 8:42 PM, David Scott wrote:

On 05/04/11 05:58, David Winsemius wrote:

On Apr 4, 2011, at 1:27 PM, Marius Hofert wrote:

Dear David,

do you know how to get plotmath-like symbols in both rows?
I tried s.th. like:

lab<- expression(paste(alpha==1, ", ", beta==2, sep=""))
xlab<- substitute(expression( atop(lab==lab., bold(foo)) ),
list(lab.=lab))
xyplot(0 ~ 0, xlab = xlab)

I _did_ have plotmath functions in both rows: But here is your
solution:

xyplot(0 ~ 0,  xlab =
     expression( atop(paste(alpha==1, "   ", beta==2), bold(bla) )) )
       )

Note that `paste` in plotmath is different than `paste` in regular
R.
It has no `sep` argument. I did try both substitute and bquote on
you
externally expression,  but lattice seems to be doing some non-
standard evaluation and I never got it to "work". Using what I
thought
_should_ work, does work with `plot`:

  x=1;y=2
  plot(0 ~ 0, xlab = bquote( atop(alpha==.(x)*","~beta==.(y),

bold(foo) ) )
+ )

But the same expression throws an error with xyplot:

  x=1;y=2
  xyplot(0 ~ 0, xlab = bquote( atop(alpha==.(x)*","~beta==.(y),

bold(foo) ) )
+ )
Error in trellis.skeleton(formula = 0 ~ 0, cond = list(1L), aspect =
"fill",  :
   could not find function "atop"

I am not sure where I read it and I can't find it again, but my
understanding is that expressions using bquote with lattice need to
be enclosed in as.expression() to work. That is in contrast to what
happens in base graphics.

Perhaps here:
http://finzi.psych.upenn.edu/Rhelp10/2010-August/250832.html

Or here:
http://finzi.psych.upenn.edu/Rhelp10/2009-July/203714.html

Although I disagree with Heimstra that reading the help(bquote)
provides "more details" that might shed light on why this is so.