Addition or Subtraction when creating a Item

[luketrot]

Its easy to take a number in zabbix and add a customer multiplier when you creat the Item but can you do the same for addition or subtraction?

[bondbig]

I'm also interested in subtraction when adding an item. Squid SMNP returns number of unused file descriptors, but i need to graph number of availiable (total is 4096). It could be simply made by subtraction: 4096 - $value, but this option is not availiable in zabbix 1.4.5