make a custom class for the burndown data toggle

app/assets/javascripts/burndown_chart/index.js

@@ -25,7 +25,7 @@ $(() => {
     let currentView = 'count';
     chart.setData(openIssuesCount, { label: 'Open issues', animate: true });
 
-    $('.burndown-header').on('click', '.btn', function switchData() {
+    $('.js-burndown-data-selector').on('click', 'button', function switchData() {
       const $this = $(this);
       const show = $this.data('show');
       if (currentView !== show) {

app/views/shared/milestones/_burndown.html.haml

@@ -11,7 +11,7 @@
   .burndown-header
     %h3
       Burndown chart
-      .btn-group
+      .btn-group.js-burndown-data-selector
         %button.btn.btn-xs.active{ data: { show: 'count' } }
           Issues
         %button.btn.btn-xs{ data: { show: 'weight' } }