Document getTimeDifferenceMinutes usage

Also added a finally clause to the getGraphsDataWithTime promise

app/assets/javascripts/monitoring/components/dashboard.vue

@@ -178,11 +178,12 @@ export default {
         .then(data => {
           this.store.storeMetrics(data);
           this.selectedTimeWindow = this.timeWindows[timeFrame];
-          this.showEmptyState = false;
         })
         .catch(() => {
-          this.showEmptyState = false;
           Flash(s__('Metrics|Not enough data to display'));
+        })
+        .finally(() => {
+          this.showEmptyState = false;
         });
     },
     onSidebarMutation() {

app/assets/javascripts/monitoring/utils.js

 import { timeWindows, msPerMinute } from './constants';
 
+/**
+ * method that converts a predetermined time window to minutes
+ * defaults to 8 hours as the default option
+ * @param {String} timeWindow - The time window to convert to minutes
+ * @returns {number} The time window in minutes
+ */
 const getTimeDifferenceMinutes = timeWindow => {
   switch (timeWindow) {
     case timeWindows.thirtyMinutes:

spec/javascripts/monitoring/dashboard_spec.js

@@ -224,7 +224,7 @@ describe('Dashboard', () => {
       });
     });
 
-    it('rewnders the time window dropdown with a set of options', done => {
+    it('renders the time window dropdown with a set of options', done => {
       const component = new DashboardComponent({
         el: document.querySelector('.prometheus-graphs'),
         propsData: {