[R] lattice: how to "center" a subtitle?

[David Scott]

On 6/04/2011 12:47 a.m., Deepayan Sarkar wrote:
> On Tue, Apr 5, 2011 at 6:12 AM, David Scott<d.scott at auckland.ac.nz>  wrote:
>
> [...]
>
>> 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 .....
>
> Let's see: ?xyplot says
>
>            'main': Typically a character string or expression describing
>                the main title to be placed on top of each page. [...]
>
> So, lattice is fairly explicit, by R standards, in requiring 'main' to
> be "character" or "expression". On the other hand, ?title says
>
>       The labels passed to 'title' can be character strings or language
>       objects (names, calls or expressions), or [...]
>
> so it additionally accepts "names" and "calls".
>
> Now, we have
>
>> a<- 2
>> foo<- bquote(alpha == .(a))
>
>> foo # Looks OK
> alpha == 2
>> mode(foo) # But
> [1] "call"
>> is.expression(foo) # not an expression
> [1] FALSE
>
>> is.expression(expression(foo)) ## YES, but
> [1] TRUE
>> expression(foo) ## not what we want
> expression(foo)
>
>> is.expression(as.expression(foo))
> [1] TRUE
>> as.expression(foo) ## This IS what we want
> expression(alpha == 2)
>
> So I submit that lattice is behaving exactly as suggested by its documentation.
>
> Now you would naturally argue that this is hiding behind
> technicalities, and if "call" objects work for plot(), it should work
> for lattice as well. But watch this:
>
>> plot(1:10, main = foo) # works perfectly
>
>> arglist<- list(1:10, main = foo)
>> arglist # Looks like what we want
> [[1]]
>   [1]  1  2  3  4  5  6  7  8  9 10
>
> $main
> alpha == 2
>
>> do.call(plot, arglist)
> Error in as.graphicsAnnot(main) : object 'alpha' not found
>
> ...which I would say is "unexpected" behaviour, if not a bug.
>
> The moral of the story is that unevaluated calls are dangerous objects
> (try this one out for fun:
>
> foo<- bquote(q(.(x)), list(x = "no"))
> do.call(plot, list(1:10, main = foo))
>
> ), and carrying them around is not a good idea.
>
> Lattice does use the do.call paradigm quite a bit, and I think it
> might be quite difficult to fix it up to handle non-expression
> language objects (which will still not fix the type of problem shown
> above).
>
> -Deepayan

Thanks very much for this explanation Deepayan. Part of my intention in 
contributing to this thread was to have something explicit in the 
archives for future reference, and your reply is excellent in that regard.

And many thanks for your work on lattice.

David Scott

-- 
_________________________________________________________________
David Scott	Department of Statistics
		The University of Auckland, PB 92019
		Auckland 1142,    NEW ZEALAND
Phone: +64 9 923 5055, or +64 9 373 7599 ext 85055
Email:	d.scott at auckland.ac.nz,  Fax: +64 9 373 7018

Director of Consulting, Department of Statistics