Graphing with Virtual Feeds not stacking up

I’m trying to make some stacked graphs, but they are not adding up (off by a reasonable amount).

My simplistic setup does this:

Which should be a positive feed and a virtual feed that is it’s negative.
Clearly there is a substantial difference between the two.
The virtual feed is as follows:

So as far as I can tell - this should be ok.

My setup is as follows:

Server Information
Emoncms Version 9.8.31 : 2018.06.21
Modules Administration : App v1.1.1 : Dashboard v1.3.0 : EventProcesses : Feed : Graph v1.2.0 : Input : CoreProcess : Schedule : Time : User : Visualisation
Server OS Linux 4.4.0-134-generic
Host 635221db22b9 635221db22b9 (192.168.144.4)
Date 2019-03-23 16:12:05 CET
Uptime 15:12:05 up 185 days, 19:40, 0 users, load average: 0.00, 0.02, 0.00
HTTP Server Apache/2.4.25 (Debian) HTTP/1.1 CGI/1.1 8080
MySQL Version 5.6.41
Host db (192.168.144.2)
Date 2019-03-23 15:12:05 (UTC 00:00‌​)
Stats Uptime: 13833464 Threads: 1 Questions: 5693231 Slow queries: 0 Opens: 79 Flush tables: 1 Open tables: 70 Queries per second avg: 0.411
Redis Version 3.0.7
Host redis:6379 (192.168.144.3)
Size 56 keys (806.37K)
Uptime 160 days
Memory RAM Used: 21.94% Total: 7.69 GB Used: 1.69 GB Free: 6 GB
Swap Used: 0.44% Total: 7.89 GB Used: 35.58 MB Free: 7.86 GB
Disk Mount Stats
/dev/shm Used: 0.00% Total: 64 MB Used: 0 B Free: 64 MB
/etc/hosts Used: 9.01% Total: 108.51 GB Used: 9.78 GB Free: 93.2 GB
PHP Version 7.0.32 (Zend Version 3.0.0)
Modules apache2handler : Core v7.0.32 : ctype v7.0.32 : curl v7.0.32 : date v7.0.32 : dom v20031129 : fileinfo v1.0.5 : filter v7.0.32 : ftp v7.0.32 : gettext v7.0.32 : hash v1.0 : iconv v7.0.32 : json v1.4.0 : libxml v7.0.32 : mbstring v7.0.32 : mcrypt v7.0.32 : mosquitto v0.4.0 : mysqli v7.0.32 : mysqlnd vmysqlnd 5.0.12-dev - 20150407 - $Id: b5c5906d452ec590732a93b051f3827e02749b83 $ : openssl v7.0.32 : pcre v7.0.32 : PDO v7.0.32 : pdo_sqlite v7.0.32 : Phar v2.0.2 : posix v7.0.32 : redis v3.1.6 : Reflection v7.0.32 : session v7.0.32 : SimpleXML v7.0.32 : SPL v7.0.32 : sqlite3 v7.0.32 : standard v7.0.32 : tokenizer v7.0.32 : xml v7.0.32 : xmlreader v7.0.32 : xmlwriter v7.0.32 : zlib v7.0.32

Client Information
HTTP Browser Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_3) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/12.0.3 Safari/605.1.15
Screen Resolution 1440 x 900
Window Size 795 x 724

[See Virtual Feeds Problems - #3 by wh01 - Moderator (RW)]