[R] sub- and superscript in plot labels

[Thomas Lumley]

On Thu, 4 Nov 2004, Gavin Simpson wrote:
>
> 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?

phantom is not part of the R language, just part of the language that 
plotmath understands (like frac and scriptstyle).  The relevant help page 
is ?plotmath.

As to why it works: phantom() returns an empty box large enough to hold 
its argument.  When given no argument, it returns a very small empty box.

In this example the importance of phantom() is that 2-phantom() is parsed 
as a binary operation, indicating that the - character should be typeset 
as a minus sign.   Writing 2- is a parse error, and writing "2-" indicates 
that the - character is ordinary text and should be typeset as a 
hyphen.

 	-thomas