report: render barchart with requested bar colors

A report can specify the bar colors of a barchart with a call to:
   (gnc:html-barchart-set-col-colors! chart (list "blue" "red"))
However, these colors were ignored when rendering the barchart.

This patch adds the jqplot option "seriesColors" to pass
the requested bar colors to jqplot. If no colors are requested,
we let jqplot use default colors by setting "seriesColors: false".

gnucash/report/report-system/html-barchart.scm

@@ -353,8 +353,10 @@
                       (gnc:html-barchart-row-labels barchart)))
          (col-labels (catenate-escaped-strings 
                       (gnc:html-barchart-col-labels barchart)))
-         (col-colors (catenate-escaped-strings 
-                      (gnc:html-barchart-col-colors barchart)))
+         ;; convert color list to string with valid js array of strings, example: "\"blue\", \"red\""
+         (colors-str (string-join (map (lambda (color)
+					 (string-append "\"" color "\""))
+				       (gnc:html-barchart-col-colors barchart)) ", "))
          (series-data-start (lambda (series-index)
                          (push "var d")
                          (push series-index)
@@ -476,6 +478,7 @@
                        showTooltip: false,
                        zoom: true,
                    },
+                   seriesColors: false,
                 };\n")
 
             (push "  options.stackSeries = ")
@@ -506,6 +509,13 @@
                 (push y-label)
                 (push "\";\n")))
             (push "  options.axes.xaxis.ticks = all_ticks;\n")
+            (if (not (equal? colors-str ""))
+                (begin            ; example: options.seriesColors= ["blue", "red"];
+                  (push "options.seriesColors = [")
+                  (push colors-str)
+                  (push "];\n")
+                  )
+                )
 
 
             (push "$.jqplot.config.enablePlugins = true;\n")