dotplot (lattice) with panel.segments and groups

On 7/7/06, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:

Could you explain what panel.groups= does and what the difference
is between panel.groups= and panel= ?  In ?xyplot it just says:

panel.groups: useful mostly for 'xyplot' and 'densityplot'. Applies
when 'panel' is 'panel.superpose' (which happens by default
in these cases if 'groups' is non-null)

which indicates when it might apply but not what it does.

That's wrong (it used to be right - a good example of why \synopsis is
bad). Since lattice 0.13-x, panel.superpose is never the default panel
function. An updated version with improved documentation should be out
soon.

'panel.groups' is simply an argument to panel.superpose, and is
described in ?panel.superpose. Thus, it only makes sense as an
argument to xyplot/dotplot/whatever when the panel function is
panel.superpose, and not otherwise. The entry for the graphical
parameters in ?panel.superpose isn't as useful as it could be, I have
just updated it to read:

col, col.line, col.symbol, pch, cex, fill, font, fontface,
fontfamily, lty, lwd, alpha: graphical
parameters, replicated to be as long as the number of
groups.  These are eventually passed down to 'panel.groups',
but as scalars rather than vectors.  When 'panel.groups' is
called for the i-th level of 'groups', the corresponding
element of each graphical parameter is passed to it.

Hope that makes things a bit clearer.