Attempting to plot a rug (using `rug()`) fails silently when attempting to use alpha-transparent colors. For example:
x <- rnorm(100)
y <- rnorm(100)
rug(x, col = rgb(1,0,0,.5))
This results in no rug being drawn and no error or warning.
The reason, I guess, is that transparent colors are not allowed. Best solution would be to allow transparent colors, but short of that this should at least issue a warning.