Fix burnup guideline

Fix missing burnup guideline on milestone start date.
Fix burnup guieline for milestones that have zero issues
on start date.

ee/app/assets/javascripts/burndown_chart/components/burndown_chart.vue

@@ -76,8 +76,10 @@ export default {
         },
       ];
 
-      if (series[0] && series[0].data.length >= 2) {
-        const idealStart = [this.startDate, data[0][1]];
+      if (data.length > 0) {
+        const zeroStart = [this.startDate, 0];
+        const firstNonZero = data.find((dataObj) => dataObj[1] !== 0) || zeroStart;
+        const idealStart = [this.startDate, firstNonZero[1]];
         const idealEnd = [this.dueDate, 0];
         const idealData = [idealStart, idealEnd];
 

ee/spec/frontend/burndown_chart/components/burndown_chart_spec.js

@@ -35,10 +35,10 @@ describe('burndown_chart', () => {
     expect(findChart().exists()).toBe(true);
   });
 
-  describe('with single point', () => {
+  describe('with no points', () => {
     it('does not show guideline', () => {
       createComponent({
-        openIssuesCount: [{ '2019-08-07T00:00:00.000Z': 100 }],
+        openIssuesCount: [],
       });
 
       const data = wrapper.vm.dataSeries;
@@ -47,6 +47,30 @@ describe('burndown_chart', () => {
     });
   });
 
+  describe('with single point and zero issues', () => {
+    it('shows guideline', () => {
+      createComponent({
+        openIssuesCount: [{ '2019-08-07T00:00:00.000Z': 0 }],
+      });
+
+      const data = wrapper.vm.dataSeries;
+      expect(data).toHaveLength(2);
+      expect(data[1].name).toBe('Guideline');
+    });
+  });
+
+  describe('with single point', () => {
+    it('shows guideline', () => {
+      createComponent({
+        openIssuesCount: [{ '2019-08-07T00:00:00.000Z': 100 }],
+      });
+
+      const data = wrapper.vm.dataSeries;
+      expect(data).toHaveLength(2);
+      expect(data[1].name).toBe('Guideline');
+    });
+  });
+
   describe('with multiple points', () => {
     beforeEach(() => {
       createComponent({