Display groups with different borders in histogram with panel.superpose

Question

This answer shows how to use groups and panel.superpose to display overlapping histograms in the same panel, assigning different colors to each his

Answer 1

panel.histogram() doesn't have a formal groups= argument, and if you examine its code, you'll see that it handles any supplied groups= argument differently and in a less standard way than panel.*() functions that do. The upshot of that design decision is that (as you've found) it's not in general easy to pass in to it vectors of graphical parameters specifying per-group appearance

As a workaround, I'd suggest using latticeExtra's +() and as.layer() functions to overlay a number of separate histogram() plots, one for each group. Here's how you might do that:

library(lattice)
library(latticeExtra)

## Split your data by group into separate data.frames
foo.df <- data.frame(x=c(rnorm(10),rnorm(10)+2), cat=c(rep("A", 10),rep("B", 10)))
foo.A <- subset(foo.df, cat=="A")
foo.B <- subset(foo.df, cat=="B")

## Use calls to `+ as.layer()` to layer each group's histogram onto previous ones  
histogram(~ x, data=foo.A, ylim=c(0,75), breaks=seq(-3, 5, 0.5), 
          lwd=2, col="transparent", border="black") +
as.layer(
histogram(~ x, data=foo.B, ylim=c(0,75), breaks=seq(-3, 5, 0.5), 
          lwd=2, col="cyan", border="red") 
)