[R] Adding reference lines to xyplot

[Damon Wischik]

> I'm trying to add a set of reference lines to a multipanel xyplot
> xyplot(y ~ x | Visit,
> 	panel = function(x, y, ...){
> 	panel.xyplot(x, y, ...)
> 	abline(v = c(0.5, 1))
> 	})
> However, the reference lines are different for different visits.

I would do it with the groups parameter.

xyplot (y ~ x | Visit,
  groups=Visit,
  panel=function(x,y,subscripts,groups,...) {
    panel.xyplot(x,y,...)
    g <- unique(groups[subscripts])
    # g is the unique value of Visit occuring in this panel
    # so plot whatever lines you like, as a function of g
    }
  )

The groups argument to xyplot is an arbitrary vector of the same length
as the data you are plotting. The function xyplot splits the data into
panels. To each panel function it passes two arguments: group, which
is just the same as the groups argument passed to xyplot; and 
subscripts, a vector of integers indexing the entries that will appear
in this panel.

Since in this case groups=Visit and the plot conditions on Visit,
groups[subscripts] contains identical elements. If you had passed
groups=SomethingElse to xyplot, groups[subscripts] would contain
several values.

> Any suggestions?  Which page of the various manuals did I skim too quickly?

The documentation I worked from is that for xyplot. And I looked
at the definition of panel.superpose.

Damon Wischik.