(defun half-normal (x &key (title "Half-Normal Plot") point-labels)
"Args: (data &key (title \"Half-Normal Plot\") point-labels)
Produces a half-normal probability plot of the data values.
This is a plot of the absolute values of the data versus their expected
order statistics from a standard normal distribution."
  (let ((adata (abs x)))
    (plot-points (sqrt (chisq-quant (/ (1+ (rank adata))
				       (1+ (length adata))) 1))
		 adata
		 :title title
		 :axis-labels
		 '("Theoretical Quantiles" "Observed Magnitudes")
		 :point-labels point-labels)))