Patch : Adding average of multiple graph items

[jfbus]

I wanted to be able to monitor the load-average of a few server pools in a single graph.

cacti's CDEFs are unable to generate such graph, so I created a new one : AVERAGE_PREVIOUS_NODUPS.

It enables the creation of such graph :

Graph Item #1 : DS#1 (LINE1, no color)
Graph Item #2 : DS#2 (LINE1, no color)
Graph Item #3 : DS#3 (LINE1, no color)
Graph Item #4 : CDEF AVERAGE_PREVIOUS (LINE2, color)
Graph Item #5 : DS#4 (LINE1, no color)
Graph Item #6 : DS#5 (LINE1, no color)
Graph Item #7 : DS#6 (LINE1, no color)
Graph Item #8 : CDEF AVERAGE_PREVIOUS (LINE2, color)

Graph Item #4 will display the average of DS#1, DS#2, DS#3.
Graph Item #8 will display the average of DS#4, DS#5, DS#6.

Installation :
- patch your cacti install
- create a CDEF (Graph Management>CDEFs), with 1 CDEF Item (Special Data Source: AVERAGE_PREVIOUS_NODUPS)

This is my first cati patch, it is certainly not perfect...