Unstack series on the "Cost Per Service Per Hour" graph

By default series on the "Cost Per Service Per Hour" graph are stacked
together which means:

* all series have to have the same length, otherwise Rickshaw/D3 won't
  be able to stack them up;
* series are drawn one over another and since they are area graphs, this
  gives false impression of scale (series other than the first one will
  appear higher than what the scale on the Y axis suggests)

This change disables stacking of data series.

Story: 2008582
Task: 41735

Change-Id: I6aed92383a70f8dcf26c45155ef1d16169d40601

cloudkittydashboard/dashboards/project/reporting/templates/reporting/this_month.html

@@ -112,6 +112,7 @@
   var graph = new Rickshaw.Graph({
     element: document.querySelector('#cost_progress'),
     interpolation: 'linear',
+    unstack: 'true',
     onComplete: function(w) {
       var legend = new Rickshaw.Graph.Legend({
         element: document.querySelector('#cost_progress_legend'),

releasenotes/notes/unstack-cost-per-service-per-hour-eff50255d66029c8.yaml

@@ -0,0 +1,4 @@
+---
+upgrade:
+  - |
+    The "Cost Per Service Per Hour" graph no longer stacks series on the Y axis.