# bland altman plot
ba_ggplot <- function(m1, m2, ...) {
# m1 and m2 are the measurements and the input arguments of the funtion
	require(ggplot2)
	means <- (m1 + m2) / 2
	diffs <- m1 - m2
	mdiff <- mean(diffs, na.rm = TRUE)
	sddiff <- sd(diffs, na.rm = TRUE)
	# Compute the figure limits
	ylimh <- mdiff + 3 * sddiff
	yliml <- mdiff - 3 * sddiff
	# Plot data
	ggplot() + 
	scale_x_continuous("Mean measurement values") +
	scale_y_continuous("Differences", limits = c(yliml, ylimh)) +
	geom_hline(yintercept = mdiff, colour = "grey", size = 2) +
	geom_hline(yintercept =  mdiff + 1.96 * sddiff, linetype = "dashed", colour = "grey", size = 2) +
	geom_hline(yintercept =  mdiff - 1.96 * sddiff, linetype = "dashed", colour = "grey", size = 2) +
	geom_point(aes(x = means, y = diffs))  # plot this last so the grey line is under it!!!
}