moved visualizing_correlation_matrices into the echarts chapter

notebooks/chapters.edn

@@ -16,8 +16,7 @@
              "interactions_ols"]}
  {:part "Data Visualization"
   :chapters ["tableplot_datavis_intro"
-             "echarts"
-             "visualizing_correlation_matrices"]}
+             "echarts"]}
  {:part "Linear Algebra"
   :chapters ["linear_algebra_intro"
              "fastmath_vector_geom2d3d"

notebooks/noj_book/echarts.clj

@@ -1,6 +1,6 @@
 ;; # Data Visualization with Echarts
 
-;; author: Zuzhi Hu, Ken Huang
+;; author: Zuzhi Hu, Ken Huang, Daniel Slutsky
 
 ;; In this tutorial, we explore how we can use
 ;; [Apache Echarts](https://echarts.apache.org/)
@@ -545,3 +545,69 @@ reshaped-stocks
 
 ;; Note the slider control and the tooltips.
 
+;; ### Correlation heatmaps
+
+;; Let us demonstrate how one may visualize [correlation matrices](https://en.wikipedia.org/wiki/Correlation#Correlation_matrices).
+;; The following auxiliary functions are inspired by the discussion at the [Clojurians Zulip chat](https://scicloj.github.io/docs/community/chat/):
+;; [#**data-science>correlation matrix plot ?**](https://clojurians.zulipchat.com/#narrow/stream/151924-data-science/topic/correlation.20matrix.20plot.20.3F)
+
+;; Rounding numbers:
+
+(defn round
+  [n scale rm]
+  (.setScale ^java.math.BigDecimal (bigdec n)
+             (int scale)
+             ^RoundingMode (if (instance? java.math.RoundingMode rm)
+                             rm
+                             (java.math.RoundingMode/valueOf
+                              (str (if (ident? rm) (symbol rm) rm))))))
+
+;; For example (see [RoundingMode](https://docs.oracle.com/javase/8/docs/api/java/math/RoundingMode.html))
+
+(round (/ 2.0 3) 2 :DOWN)
+(round (/ 2.0 3) 2 :UP)
+(round (/ 2.0 3) 2 :HALF_EVEN)
+
+;; Computing a correlation matrix and representing it as a dataset:
+
+(defn correlations-dataset [data columns-to-use]
+  (let [matrix (->> columns-to-use
+                    (mapv #(get data %))
+                    fastmath.stats/correlation-matrix)]
+    (->> matrix
+         (map-indexed
+          (fn [i row]
+            (let [coli (columns-to-use i)]
+              (->> row
+                   (map-indexed
+                    (fn [j corr]
+                      (let [colj (columns-to-use j)]
+                        {:i i
+                         :j j
+                         :coli coli
+                         :colj colj
+                         :corr corr
+                         :corr-round (round corr 2 :HALF_EVEN)})))))))
+         (apply concat)
+         tc/dataset)))
+
+;; For example:
+(-> noj-book.datasets/iris
+    (correlations-dataset [:sepal-length :sepal-width :petal-length :petal-width]))
+
+;; Visualizing a corrleation matrix as a heatmap:
+
+(let [columns-for-correlations [:sepal-length :sepal-width
+                                :petal-length :petal-width]
+      correlations (-> noj-book.datasets/iris
+                       (correlations-dataset columns-for-correlations)
+                       (tc/select-columns [:coli :colj :corr-round])
+                       tc/rows)]
+  (echarts-heatmap {:xyz-data correlations
+                    :xs columns-for-correlations
+                    :ys columns-for-correlations
+                    :min -1
+                    :max 1
+                    :series-name "correlation"}))
+
+