source("http://192.38.117.59/~linearpredictors/datafiles/readVitaminD.R")

irlwomen$bmigroup3 <- cut(irlwomen$bmi, c(0,25, 30, Inf), right = F, labels = 0:2) 

xbar <- tapply(irlwomen$vitd, irlwomen$bmigroup3, mean)

## Scatterplot of Vitamin D against the 3 bmi groups.
offset <- c(5/4, 2, 11/4)
stripchart(irlwomen$vitd ~ irlwomen$bmigroup3,
           vertical = TRUE,
           pch = 21,
           method = "jitter", jitter = 0.01,
           group.names = c("","",""),
           xlab = "BMI",
           ylab = "Vitamin D",
           at = offset ## where to place groups at x axis
           )
axis(1, at = offset, labels = expression("" < 25, 25-30, "" >= 30))

## marking groupwise means of Vitamin D
r <- 0.04
segments(offset-r, xbar, offset+r, xbar)