[R] ggplot2: mapping categorical variable to color aesthetic with faceting

[Bryan Hanson]

Hello Again...  I¹m making a faceted plot of a response on two categorical
variables using ggplot2 and having troubles with the coloring. Here is a
sample that produces the desired plot:

compareCats <- function(data, res, fac1, fac2, colors) {

    require(ggplot2)
    p <- ggplot(data, aes(fac1, res)) + facet_grid(. ~ fac2)
    jit <- position_jitter(width = 0.1)
    p <- p + layer(geom = "jitter", position = jit, color = colors)
    print(p)
    }

test <- data.frame(res = rnorm(100), fac1 = as.factor(rep(c("A", "B"), 50)),
    fac2 = as.factor(rep(c("lrg", "lrg", "sm", "sm"), 25)))

compareCats(data = test, res = res, fac1 = fac1, fac2 = fac2, colors =
c("red", "blue"))

Now, if I get away from idealized data where there are the same number of
data points per group (25 in this case), I run into problems.  So, if you
do:

rem <- runif(5, 1, 100) # randomly remove a few points here and there
test <- test[-rem,]
compareCats(data = test, res = res, fac1 = fac1, fac2 = fac2, colors =
c("red", "blue"))

R throws an error due to mismatch between the recycling of colors and the
actual number of data points:

Error in `[<-.data.frame`(`*tmp*`, gp, value = list(colour = c("red",  :
  replacement element 1 has 2 rows, need 47

I'm new to ggplot2, but have been through the book and the web site enough
to know that my problem is "mapping the varible to the aesthetic"; I also
know I can either "map" or "set" the colors.

The question, finally:  is there an simple/elegant way to map a list of two
colors corresponding to A and B onto any random sample size of A and B with
faceting?  If not, and I must "set" the colors:  Do I compute the length of
all possible combos of A, B with lrg, sm, and then create one long vector of
colors for the entire plot?  I tried something like this, and was not
successful, but perhaps could be with more work.

All advice appreciated, Bryan (session info below)

*************
Bryan Hanson
Professor of Chemistry & Biochemistry
DePauw University, Greencastle IN USA

> sessionInfo()
R version 2.9.2 (2009-08-24)
i386-apple-darwin8.11.1

locale:
en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] grid      datasets  tools     utils     stats     graphics  grDevices
methods  
[9] base     

other attached packages:
 [1] ggplot2_0.8.3      reshape_0.8.3      proto_0.3-8        mvbutils_2.2.0
 [5] ChemoSpec_1.1      lattice_0.17-25    mvoutlier_1.4      plyr_0.1.8
 [9] RColorBrewer_1.0-2 chemometrics_0.4   som_0.3-4
robustbase_0.4-5  
[13] rpart_3.1-45       pls_2.1-0          pcaPP_1.7          mvtnorm_0.9-7
[17] nnet_7.2-48        mclust_3.2         MASS_7.2-48        lars_0.9-7
[21] e1071_1.5-19       class_7.2-48