(defmacro time-series-plot
" Returns a JFreeChart object representing a time series plot of the given data.
  Use the 'view' function to display the chart, or the 'save' function
  to write it to a file. Sequence passed in for the x axis should be
  number of milliseconds from the epoch (1 Janurary 1970).

  Options:
    :data (default nil) If the :data option is provided a dataset, 
                        column names can be used instead of sequences 
                        of data as arguments to xy-plot.
    :title (default 'Time Series Plot') main title
    :x-label (default x expression)
    :y-label (default y expression)
    :legend (default false) prints legend
    :series-label (default x expression)
    :group-by (default nil) -- a vector of values used to group the x and y values into series.

  See also:
    view, save, add-points, add-lines

  Examples:

    (use '(incanter core stats charts chrono))

    ;; plot numbers against years starting with 1900 
    (def dates (map #(-> (joda-date (+ 1900 %) 1 1 12 0 0 0 (time-zone 0)) 
                         .getMillis) 
                    (range 100)))
    (def y (range 100))
    (view (time-series-plot dates y
                            :x-label \"Year\"))

  References:
    http://www.jfree.org/jfreechart/api/javadoc/
    http://www.jfree.org/jfreechart/api/javadoc/org/jfree/chart/JFreeChart.html

"
  ([x y & options]
    `(let [opts# ~(when options (apply assoc {} options))
           group-by# (:group-by opts#) 
           main-title# (or (:title opts#) "")
           x-lab# (or (:x-label opts#) (str '~x))
           y-lab# (or (:y-label opts#) (str '~y))
           series-lab# (or (:series-label opts#) (if group-by#
						   (format "%s, %s (0)" '~x '~y) 
						   (format "%s, %s" '~x '~y)))
	   args# (concat [~x ~y ~create-time-series-plot] (apply concat (seq (apply assoc opts# 
							   [:group-by group-by# 
							    :main-title main-title# 
							    :x-label x-lab# 
							    :y-label y-lab# 
							    :series-label series-lab#]))))]
        (apply xy-plot* args#))))