add header to burndown chart and adjust chart size breakpoints

app/assets/stylesheets/pages/milestone.scss

@@ -253,17 +253,26 @@ $burndown-chart-legend-stroke: #e5e5e5;
 $burndown-chart-axis-label-color: #5c5c5c;
 $burndown-chart-axis-label-stroke: #e5e5e5;
 
+.burndown-header {
+  margin: 24px 0 12px;
+
+  h3 {
+    font-size: 16px;
+    margin: 0;
+  }
+}
+
 .burndown-chart {
   width: 100%;
-  height: 400px;
-  margin-top: 30px;
+  height: 380px;
+  margin: 5px 0;
 
   @media (max-width: $screen-sm-max) {
     height: 320px;
   }
 
   @media (max-width: $screen-xs-max) {
-    height: 220px;
+    height: 200px;
   }
 
   .axis {

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

@@ -22,4 +22,7 @@
         = link_to "Add start and due date", edit_namespace_project_milestone_path(project.namespace, project, milestone), class: 'btn'
 
 - if can_generate_chart
+  .burndown-header
+    %h3
+      Open issues
   .burndown-chart