Fix pnp4nagios graphs for memory check
rrdtool chokes with "No DS called ...": https://unstable.bitmask.net/pnp4nagios/index.php/graph?host=aardwolf.unstable.bitmask.i&srv=Memory%20used
(from redmine: created on 2016-05-26)
rrdtool chokes with "No DS called ...": https://unstable.bitmask.net/pnp4nagios/index.php/graph?host=aardwolf.unstable.bitmask.i&srv=Memory%20used
(from redmine: created on 2016-05-26)