lattice: how to "center" a subtitle?

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.

Here is a simple example.

a <- 2
plot(1:10, a*(1:10), main = bquote(alpha == .(a)))
require(lattice)
xyplot(a*(1:10)~ 1:10, main = bquote(alpha == .(a)))
xyplot(a*(1:10)~ 1:10, main = as.expression(bquote(alpha == .(a))))

Which produces:

a <- 2
plot(1:10, a*(1:10), main = bquote(alpha == .(a)))
require(lattice)

Loading required package: lattice

xyplot(a*(1:10)~ 1:10, main = bquote(alpha == .(a)))

Error in trellis.skeleton(formula = a * (1:10) ~ 1:10, cond =  
list(c(1L,  :
 object 'alpha' not found

xyplot(a*(1:10)~ 1:10, main = as.expression(bquote(alpha == .(a))))

Using expression() rather than as.expression() doesn't produce the  
desired affect. Try it yourself.

As to why this is the case .....