Up to Artisan v0.9.7 custom event buttons could only set absolute event values. The version 0.9.8 adds the possibility to define buttons that increase or decrease an event value relative to the last one registered.
To define such a "relative" event button, set its type to one of the available types from the popup prefixed by +/- and put the intended offset as value (e.g. -5). You can also set an event action if needed and put potentially a command to be send (e.g. for the case of MODBUS or Serial actions) in the description field. Any placeholder of the form {} in that description will be replaced by the new event value (and not by the offset value specified).
For example, instead of the extra event buttons to set the heater and fan of a Hottop KN-8828B-2k+ to absolute values, one could use two buttons to increase and decrease the heater as well as two buttons to increase and decrease the fan by 10%.
