Difference in values at each interval in a series

Append a new series showing the difference in values at each interval

HTML

<script src="https://d26b395fwzu5fz.cloudfront.net/3.4.0/keen.min.js?date=2016-02-17"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/c3/0.4.10/c3.min.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.6/d3.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/c3/0.4.10/c3.min.js"></script>
<div id="chart"></div>

JavaScript

var client = new Keen({
    projectId: "5011efa95f546f2ce2000000",
    readKey: "ef717eaec4aeb8e8b8b18891ffaa1eafed8c14cac1f7cb90030eaa7ed79ed2d540ee267547267c73f4c6e7ff9bcb8f7dec4df9500499a70737777d71971a20e6e3cd6532b44a44c5d269811178c2308867fd5082d44e51c05496b0099c4a06649207b8db43fe0458cf3cc059f5ecaadc"
});

Keen.ready(function(){
    var after_project_page = new Keen.Query("count", {
        eventCollection: "pageviews",
        timeframe: 'this_4_weeks',
        interval: "daily",
        filters: [
        	{
          	property_name: 'recentInternalReferrer.relative',
            operator: 'contains',
            property_value: '/project/'
          },
        	{
          	property_name: 'recentInternalReferrer.relative',
            operator: 'not_contains',
            property_value: 'explorer'
          },
        	{
          	property_name: 'recentInternalReferrer.relative',
            operator: 'not_contains',
            property_value: 'settings'
          },
        	{
          	property_name: 'recentInternalReferrer.relative',
            operator: 'not_contains',
            property_value: 'dashboard'
          },
        	{
          	property_name: 'recentInternalReferrer.relative',
            operator: 'not_contains',
            property_value: 'root'
          }
        ]
    });
    var on_project_page = new Keen.Query("count", {
        eventCollection: "pageviews",
        timeframe: 'this_4_weeks',
        interval: "daily",
        filters: [
        	{
          	property_name: 'url.relative',
            operator: 'contains',
            property_value: '/project/'
          },
        	{
          	property_name: 'url.relative',
            operator: 'not_contains',
            property_value: 'explorer'
          },
        	{
          	property_name: 'url.relative',
            operator: 'not_contains',
            property_value: 'settings'
          },
        	{
          	property_name: 'url.relative',
           ...