2.6. Performance data
Performance data is defined by Nagios as “everything after the | of the plugin output” - please refer to Nagios documentation for information on capturing this data to logfiles. However, it is the responsibility of the plugin writer to ensure the performance data is in a “Nagios plugins” format. This is the expected format:
'label'=value[UOM];[warn];[crit];[min];[max]
Notes:
-  space separated list of label/value pairs 
-  label can contain any characters 
-  the single quotes for the label are optional. Required if spaces, = or ' are in the label 
-  label length is arbitrary, but ideally the first 19 characters are unique (due to a limitation in RRD). Be aware of a limitation in the amount of data that NRPE returns to Nagios 
-   to specify a quote character, use two single quotes 
-   warn, crit, min/ or max/ may be null (for example, if the threshold is not defined or min and max do not apply). Trailing unfilled semicolons can be dropped 
-  min and max are not required if UOM=% 
-  value, min and max in class [-0-9.]. Must all be the same UOM 
-  warn and crit are in the range format (see Section 2.5). Must be the same UOM 
-  UOM (unit of measurement) is one of: - 
-  no unit specified - assume a number (int or float) of things (eg, users, processes, load averages) 
-  s - seconds (also us, ms) 
-  % - percentage 
-  B - bytes (also KB,  MB- , TB,  GB- ?) 
-  c - a continous counter (such as bytes transmitted on an interface) 
 
It is up to third party programs to convert the Nagios plugins performance data into graphs.
Origin: https://www.monitoring-plugins.org/doc/guidelines.html#AEN200