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R Quickie: Custom Panel Functions and Default Arguments

Apr 16, 2012 metroadmin

Sometimes the basic functionality in lattice graphics isn't enough. Custom "panel functions" are one approach to fully customizing the lattice graphics system. Two examples are given below illustrating how to define an (inline) custom panel function for adding a regression line to an entire data set in the presence of grouping variable. The "..." keyword instructs our custom panel function to accept all arguments typically passed to a panel function, and can be re-used in clever ways within our panel function.

library(lattice)

# simulate correlated data
n <- 200
x <- rnorm(n, mean=0, sd=1)
y <- x * rnorm(n, mean=2, sd=2) + rnorm(n, mean=5, sd=1)

# simulate two levels of grouping
g <- gl(n=4, k=25, length=n, labels=letters[1:4])
f <- gl(n=2, k=50, length=n, labels=c('g1', 'g2'))

# combine into DF
d <- data.frame(x, y, g, f)

# define a custom panel function
# note that "..." passes all of the default arguments from parent function
# to child function(s)
newPanelFunction <- function(...) {
   # normally called by xyplot() to do routine plotting
   panel.superpose(...)
   # add regression line ignoring groups
   panel.lmline(..., col='black', lwd=2, lty=2)
}

# plot y ~ x, with points, grid, and group-wise regression line
# use custom panel function, to add regression line for entire data set
xyplot(y ~ x, groups=g, data=d, type=c('p','g', 'r'),
auto.key=list(columns=4, points=TRUE, lines=FALSE),
panel=newPanelFunction)

# works seamlessly with multiple panels
xyplot(y ~ x | f, groups=g, data=d, type=c('p','g', 'r'),
auto.key=list(columns=4, points=TRUE, lines=FALSE),
panel=newPanelFunction)