[R] Groups and bwplot

[Sébastien Bihorel]

Dear R-users,

A while ago, Deepayan Sarkar suggested some code that uses the group
argument in bwplot to create some 'side-by-side' boxplots
(https://stat.ethz.ch/pipermail/r-help/2010-February/230065.html). The
example he gave was relatively specific and I wanted to generalize his
approach into a function. Unfortunately, I seem to have some issues
passing the correct arguments to the panel function, and would greatly
appreciate any suggestions to solve these issues:

require(lattice)

mybwplot <- function(x,y,data,groups){

  if (missing(groups)||is.null(groups)) {
    groups <- NULL
    ngroups <- 1
  } else {
    data[[groups]] <- as.factor(data[[groups]])
    ngroups <- nlevels(data[[groups]])
  }
  mywidth <- 1/(ngroups+1)

  mypanel <- function(x,y,groups,...){
    if (missing(groups)||is.null(groups)) {
      panel.bwplot(x,y,...)
    } else {
      panel.superpose(x,y,...)
    }
  }

  mypanel.groups <- function(x,y,groups,ngroups,...){
    if (missing(groups)||is.null(groups)){
      NULL
    } else {
      function(x, y, ..., group.number) {
        panel.bwplot(x+(group.number-0.5*(ngroups+1))/(ngroups+1),y, ...)}
    }
  }

  bwplot(formula(paste(y,x,sep=' ~ ')),
       data = data,
       groups = 'Plant',
       ngroups=ngroups,
       pch = "|",
       box.width = mywidth,
       panel = mypanel,
       panel.groups = mypanel.groups)

}

myCO2 <- CO2
myCO2$year <- 2011
summary(myCO2)

mybwplot('Type','uptake',myCO2) # works

mybwplot('Type','uptake',myCO2,'Treatment') # Error using packet 1,
'x' is missing

mybwplot('Type','uptake',myCO2,'year') # Error using packet 1, 'x' is missing

# Deepayan Sarkar suggested code (adapted to myC02)
# bwplot(uptake ~ Plant, data = myCO2, groups = Treatment,
#        pch = "|", box.width = 1/3,
#        panel = panel.superpose,
#        panel.groups = function(x, y, ..., group.number) {
#            panel.bwplot(x + (group.number-1.5)/3, y, ...)
#        })