Distinguishing a second categorical variable by color in a stripchart across groups

For this example, I generated a dummy data set containing repeated measures --- I created data frame in wide format first (y) and then used the reshape() function to reshape the data to long format (longy). The stripchart shows weight across time with color of points designating sex. I used the add argument to add points from the second stripchart to the plot of the first stripchart. y <- data.frame(id = 1:10, sex = sample(c("M", "F"), size = 10, replace = TRUE), weight.t1 = sample(150:250, size = 10), weight.t2 = sample(150:250, size = 10), weight.t3 = sample(150:250, size = 10)) longy <- reshape(y, varying = list(paste("weight.t", 1:3, sep="")), direction = "long", idvar = "id") # Stripchart: with(subset(longy, sex == "M"), stripchart(weight.t1 ~ time, method = "jitter", ylim = c(150, 250), vertical = TRUE,xlab="Time", ylab = "Weight", col = "red", pch = 20)) with(subset(longy, sex == "F"), stripchart(weight.t1 ~ time, method = "jitter", vertical = TRUE, col = "blue", pch = 20, add = TRUE)) par(xpd = NA) legend(3, 270, legend = c("Male", "Female"), col = c("red", "blue"), pch = 20)