Add initDateArray utility method

Creates a two-dimensional array of length=n
where n is the difference in days between a given
start and end date plus 1

ee/app/assets/javascripts/analytics/productivity_analytics/utils.js

-import { getDateInPast } from '~/lib/utils/datetime_utility';
+import { getDayDifference, getDateInPast } from '~/lib/utils/datetime_utility';
 import { median } from '~/lib/utils/number_utils';
 
 /**
@@ -29,6 +29,21 @@ export const getMilestonesEndpoint = (namespacePath, projectPathWithNamespace) =
   return `/groups/${namespacePath}/-/milestones`;
 };
 
+/**
+ * Computes the day difference 'days' between a given start and end date
+ * and creates an array of length 'days'.
+ * For each day in the array it initializes an empty array.
+ *
+ * E.g. initDateArray(new Date('2019-01-01'), new Date('2019-01-03'))
+ * the following data structure gets generated: [ [], [], [] ]
+ * @param {Date} startDate - The start date
+ * @param {Date} endDate - The end date
+ */
+export const initDateArray = (startDate, endDate) => {
+  const days = getDayDifference(startDate, endDate);
+  return Array.from({ length: days + 1 }, () => []);
+};
+
 /**
  * Transforms a given data object into an array
  * which will be used as series data for the scatterplot chart.

ee/spec/frontend/analytics/productivity_analytics/utils_spec.js

 import {
   getLabelsEndpoint,
   getMilestonesEndpoint,
+  initDateArray,
   getScatterPlotData,
   getMedianLineData,
 } from 'ee/analytics/productivity_analytics/utils';
@@ -35,6 +36,15 @@ describe('Productivity Analytics utils', () => {
     });
   });
 
+  describe('initDateArray', () => {
+    it('creates a two-dimensional array with 3 empty arrays for startDate=2019-09-01 and endDate=2019-09-03', () => {
+      const startDate = new Date('2019-09-01');
+      const endDate = new Date('2019-09-03');
+
+      expect(initDateArray(startDate, endDate)).toEqual([[], [], []]);
+    });
+  });
+
   describe('getScatterPlotData', () => {
     it('filters out data before given "dateInPast", transforms the data and sorts by date ascending', () => {
       const dateInPast = new Date(2019, 7, 9); // '2019-08-09T22:00:00.000Z';