[Xy-pic] Vertical alignment in xymatrix

[Michael Abbott]

> There are many diagrams for which \xymatrix is not the best environment
> to use.
>
> One alternative is to use  \xygraph .
Ah, ok.  Thank you for the pointer.  Time to take another look at it, I
guess!

> This lets you control the placement of elements much more accurately.
> But you may have to work a little bit harder to master the syntax,
Yes.  The last time I looked at xygraph I bounced off it, as the syntax
seems to be a *lot* harder to understand, and xymatrix works nicely.

> > [How] difficult will it be to create an option which computes the xy
> > box with the centre in the *right* place?
>
> Within the context of \xymatrix  yes, that is actually rather hard.
>
> Using \xygraph, or explicit Xy-pic kernel commands, this can indeed be
> done --- at the expense of a more complicated syntax.
> It also will depend on the specific definition of "right place"
> that you wish to adopt.

Umm.  Could I beg a quick helping hand then to get me started?

I started with the (artificial) example
	\xymatrix{-^X \ar[r] & -_X}
where I want the two - signs to be aligned on the *same* horzontal line
without having to perform ugly hacks on the heights and depths of the two
expressions.

How could this be written using \xygraph, please?