[R] Accessor functions in lattice graphics

[David Winsemius]

Frank Harrell wrote:
> 
> I know about the current.row, current.column, and panel.number functions
> that are useful within panel functions written for lattice.  Are there
> easy ways to obtain the names of the conditioning variables (those
> appearing after |) and their values for the current panel?
> Thanks
> Frank
> 

Using one of the examples in help(xyplot):

states <- data.frame(state.x77,
                     state.name = dimnames(state.x77)[[1]],
                     state.region = state.region)
testp <- xyplot(Murder ~ Population | state.region, data = states,
       groups = state.name,
       panel = function(x, y, subscripts, groups) {
           ltext(x = x, y = y, labels = groups[subscripts], cex=1,
                 fontfamily = "HersheySans")
       })
str(testp)

I see that the names of the state.regions are in an attribute of
testp$packet.sizes, so:

> attr(testp$packet.sizes, "dimnames")[["state.region"]][4]
#[1] "West"

#Looking for a further example in help(xyplot with 2 "panel dimensions" I
tried:

> testp2 <- dotplot(variety ~ yield | year * site, data=barley)

#I'm not quite sure how the "rows" and "columns" are defined , but I get:

> attr(testp2$packet.sizes, "dimnames")[[1]][2]
[1] "1931"
> attr(testp2$packet.sizes, "dimnames")[[2]][1]
[1] "Grand Rapids"

I'm not sure this is the "right answer" because after looking further, I now
see a list node entitled $condlevels which returns the same information is a
much more straightforward fashion:

> testp2 $ condlevels
$year
[1] "1932" "1931"

$site
[1] "Grand Rapids"    "Duluth"          "University Farm" "Morris"         
[5] "Crookston"       "Waseca"      

(And other nodes that contain information about conditioning levels:
$ index.cond       :List of 2
  ..$ : int [1:2] 1 2
  ..$ : int [1:6] 1 2 3 4 5 6
 $ perm.cond        : int [1:2] 1 2
 $ condlevels       :List of 2
  ..$ year: chr [1:2] "1932" "1931"
  ..$ site: chr [1:6] "Grand Rapids" "Duluth" "University Farm" "Morris" ...

HTH;

David.

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