sub- and superscript in plot labels

On Thu, 4 Nov 2004, Gavin Simpson wrote:

Dear List,

I need to add a subscript and a superscript to some of the ions in the 
labels on some plots.

I have got to here but now I'm stuck:

plot(1:10, xlab = expression(paste("nm SO"[4], " ", mu, "eq cm"^{-2}, " 
yr"^{-1})))

Which gives almost  what I require. No matter what I tried, however, I 
could not get bot a sub script *and* a superscript attached to the SO in 
the label.

In LaTeX I would just do $SO_4^{2-}$ but taking that to R produces a 
syntax error:

plot(1:10, xlab = expression(paste("nm SO"[4]^{2-}, " ", mu, "eq 
cm"^{-2}, " yr"^{-1})))

The problem is 2-.  That's not an R expression.  Using "2-" might give
what you want, but it will use a hyphen rather than a minus. Otherwise

plot(1:10, xlab = expression(paste("nm SO"[4]^{2-phantom()})))

will give a minus.

Many thanks to Brian Ripley and Bendix Carstensen for your replies.

Both of the above options produce what I was after. One quick follow-up 
question regarding the use of phantom(). Looking at ?plotmath phantom 
leaves space for a character passed as an argument to phantom(), but 
does not plot it.

In the example above we are leaving space for "nothing". I don't 
understand why this is a valid R expression. I guess phantom() is 
returning something that makes 2-<returned_val> a valid expression, but 
I couldn't find the help for ?phantom so I couldn't check on this in the 
documentation.

Also as an aside, phantom() appears, visually, to be a function, but it 
is not visible to the user as a function. i.e. typing phantom at the 
prompt yields: Error: Object "phantom" not found. getAnywhere(phantom) 
yields nothing either. What is phantom() in R parlance?

All the best,

Gavin

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Gavin Simpson                     [T] +44 (0)20 7679 5522
ENSIS Research Fellow             [F] +44 (0)20 7679 7565
ENSIS Ltd. & ECRC                 [E] gavin.simpson at ucl.ac.uk
UCL Department of Geography       [W] http://www.ucl.ac.uk/~ucfagls/cv/
26 Bedford Way                    [W] http://www.ucl.ac.uk/~ucfagls/
London.  WC1H 0AP.
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