(defn add-text
" Adds a text annotation centered at the given coordinates.

  Arguments:
    chart -- the chart to annotate
    x, y -- the coordinates to center the text
    text -- the text to add


  Examples:

    ;; PCA chart example
    (use '(incanter core stats charts datasets))
    ;; load the iris dataset
    (def iris (to-matrix (get-dataset :iris)))
    ;; run the pca
    (def pca (principal-components (sel iris :cols (range 4))))
    ;; extract the first two principal components
    (def pc1 (sel (:rotation pca) :cols 0))
    (def pc2 (sel (:rotation pca) :cols 1))

    ;; project the first four dimension of the iris data onto the first
    ;; two principal components
    (def x1 (mmult (sel iris :cols (range 4)) pc1))
    (def x2 (mmult (sel iris :cols (range 4)) pc2))

    ;; now plot the transformed data, coloring each species a different color
    (def plot (scatter-plot x1 x2
                            :group-by (sel iris :cols 4)
                            :x-label \"PC1\" :y-label \"PC2\" :title \"Iris PCA\"))
    (view plot)
    ;; add some text annotations
    (add-text plot -2.5 -6.5 \"Setosa\")
    (add-text plot -5 -5.5 \"Versicolor\")
    (add-text plot -8 -5.5 \"Virginica\")


"
  ([chart x y text & options]
    (let [opts (when options (apply assoc {} options))
          anno (XYTextAnnotation. text x y)]
      (.addAnnotation (.getPlot chart) anno))))