|x||The X float||float64 (required)|
|y||The Y float||float64 (required)|
Calculates the rate (derivative) between two quantities.
For example if a monitoring plugin returns an absolute value sampled in time (e.g. bytes transferred sampled every second) then the rate() plugin can calculate the average bytes/sec.
This function works by remembering the values of x and y from the previous row and applying the current rows values.