Dear useRs,
I have a question regarding panel functions in the lattice package, in
particular how to customize the plotting colors in xyplot.
I have longitudinal data from several treatments (TRT) with subjects (SUBJ)
nested in treatment. Each subject has several time points; the time points are
nearly balanced across subjects, although not completely. At each sample
(SAMP) at each time point (TIME), two replicates (REP) are taken. Let's call
the response Y and the whole data.frame DAT.
My goal is to make a trellis plot with each treatment in its own panel, each
subject having its own color and lines connecting the REPs within SAMP (after
adding jitter along the TIME axis to distinguish SUBJs). Let me switch over to
an explicit example with artificial data.
set.seed(695442223)
DAT <- data.frame(TRT=factor(rep(c("Trt1","Trt2"), each=12)), #
2 treatments
SUBJ=factor(paste('subj', rep(seq(4), each=6), sep='')), #
4 subjects, 2 w/in each TRT
SAMP=factor(rep(seq(12), each=2)), #
4x3 = 12 samples
REP=factor(c(1,2)), #
2 replicates
TIME=rep(c(4,4,8,8,12,12), times=4), #
3 arbitrary times
Y=round(rep(rgamma(n=12, shape=47500, scale=1/500),
each=2) +
rep(rgamma(n=12, shape=4, scale=1/8), each=2) *
c(-1,1),
2) # arbitrary Y but very easy to view
)
DAT <- transform(DAT,
# horizontal jitter
plotX=TIME+ifelse(SUBJ %in% c("subj1","subj3"), -1,1)/10,
# unique plotting color for each subject
COLS=factor(c( "green", "black", "blue",
"red"))[as.numeric(SUBJ)]
)
The canonical example is:
xyplot(Y ~ plotX | TRT, data = DAT, groups = SAMP, type="b")
Of course, this connects the REPs within SAMP, since SAMP is the "groups"
variable. But each SAMP is now independently colored, recycled from (if I
understand correctly) trellis.par.get('superpose.<symbol/line>')$col, instead
of each SUBJ being independently colored from the COLS variable.
In Section 5.2 in Deepayan Sarkar's "Lattice: Multivariate Data Visualization
with R" (Springer 2008), the suggestion in Section 5.2 is to use a panel
function which gets passed an explicit "subscripts" argument, to distinguish
the groups indexing from the color indexing. Starting with the book's code to
plot Figure 5.4, I hacked around to get this:
xyplot(Y ~ plotX | TRT, data = DAT, groups = SAMP, type="b",
# the full vector of colors
cols = DAT[['COLS']],
panel = function(x, y, ..., cols, groups, subscripts) {
colSUBJ <- cols[subscripts]
panel.xyplot(x, y, groups=groups,
subscripts=subscripts,
..., col.line=colSUBJ,
col.symbol=colSUBJ)
}
)
Now this gets each treatment TRT with its own color, but not each subject SUBJ.
I am indeed not exactly sure what this code does specifically, with the
subscripts argument. But nearly every other permutation of arguments I could
think off either gave a totally wrong plot, or an error.
How do I write a panel function to join reps in the same sample with the groups
argument, but give each subject a unique color? (an alternate chore is to use
only unique colors within each treatment, or repeating colors in different
treatments). And what is a good pointer or two to good resources explaining
the use of the subscripts argument in panel functions? Sarkar's book is
fantastic but its treatment of this issue is quite thin.
Thanks,
John Szumiloski
John Szumiloski, Ph.D.
Principal Scientist, Statistician
Analytical and Bioanalytical Development
NBR105-1-1411
Bristol-Myers Squibb
P.O. Box 191
1 Squibb Drive
New Brunswick, NJ
08903-0191
(732) 227-7167
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