lattice panel function to display a segmented regression

gistfile1.r

# FIXME: There's a bug where if data in one of the groups is shorter than the others, spurious predictions 
# from the segmented linear model will be repeated until the number of predictions is the same as the other
# data sequences.

panel.segmented_lm <- function(x, y, groups, subscripts, ...) 
{
  g = groups[subscripts]
  for (group in levels(g)) {
    x2 = x[g==group]
    y2 = y[g==group]
    if (length(x2) == 0 || length(y2) == 0)
      next
    lm_fit = lm(y2~x2)
    # If segmented regression fails, fall back to the simple linear regression
    segmented_fit = tryCatch(segmented(lm_fit, seg.Z=~x2, psi=list(x2=c(mean(x2)))),
                             error = function(e) {
                               lm_fit
                             })
    panel.lines(x2, predict(segmented_fit), col = "black")
  }
}