R help - Jun 2009 - lattice: horizontal alignment of labels in key

Dear R users

I have problems horizontally aligning labels in keys of lattice plots when the 
labels make use of plotmath. Here's a self-contained example:

dat <- data.frame(x = rnorm(10),
                  y = rnorm(10),
                  z = factor(rep(c("a", "b"), each = 5)))
xyplot(y ~ x, dat, groups = z,
       auto.key = list(columns = 2,
         text = c(expression(plain("alright")),
           expression(italic("too low")))))

It seems like the grid object which the lattice function creates from the key 
arguments has different heights for the two labels since the second label does 
not stretch over the baseline.

Any ideas how I could circumvent the problem (other than manipulating the key 
grob)?

Best wishes
Thomas Zumbrunn

Hi


Thomas Zumbrunn wrote:
> Dear R users
> 
> I have problems horizontally aligning labels in keys of lattice plots when
the
> labels make use of plotmath. Here's a self-contained example:
> 
> dat <- data.frame(x = rnorm(10),
>                   y = rnorm(10),
>                   z = factor(rep(c("a", "b"), each =
5)))
> xyplot(y ~ x, dat, groups = z,
>        auto.key = list(columns = 2,
>          text = c(expression(plain("alright")),
>            expression(italic("too low")))))
> 
> It seems like the grid object which the lattice function creates from the
key
> arguments has different heights for the two labels since the second label
does
> not stretch over the baseline.

I think the basic problem here is that mathematical expressions work 
with the limits of bounding boxes rather than an overall baseline.  A 
workaround is to use phantom() within the expression to introduce 
elements with a consistent height that are not actually drawn.  For 
example ...

xyplot(y ~ x, dat, groups = z,
        auto.key = list(columns = 2,
          text =
c(expression(plain("alright")*phantom("hg")),
            expression(italic("too low")*phantom("hg")))))

... which forces both expressions to go as high as the top of an "h"
and
as low as the bottom of a "g" without those characters being drawn 
(although space is allowed for those characters so you get a bit of 
blank horizontal space).

Paul

> Any ideas how I could circumvent the problem (other than manipulating the
key
> grob)?
> 
> Best wishes
> Thomas Zumbrunn
> 
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-- 
Dr Paul Murrell
Department of Statistics
The University of Auckland
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paul at stat.auckland.ac.nz
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