this graph is for all memory available to processes. this is just a snippet for mrtg:

Target[test.mem]: .1.3.6.1.4.1.2021.4.6.0&.1.3.6.1.4.1.2021.4.6.0:comname@ip + .1.3.6.1.4.1.2021.4.14.0&.1.3.6.1.4.1.2021.4.14.0:comname@ip + .1.3.6.1.4.1.2021.4.15.0&.1.3.6.1.4.1.2021.4.15.0:comname@ip
PageTop[test.mem]: <H1>Free Memory</H1>
Options[test.mem]: nopercent,growright,gauge,noinfo
Title[test.mem]: Free Memory
MaxBytes[test.mem]: 1000000000000
kMG[test.mem]: k,M,G,T,P,X
YLegend[test.mem]: Memory
ShortLegend[test.mem]: bytes
LegendI[test.mem]: Total Free:
Legend2[test.mem]:
LegendO[test.mem]: Total Free:
Legend1[test.mem]: Free memory, not including swap, in bytes
# memAvailReal.0 = .1.3.6.1.4.1.2021.4.6.0
# memBuffer.0 = .1.3.6.1.4.1.2021.4.14
# memCached.0 = .1.3.6.1.4.1.2021.4.15

replace comname@ip with your community name @ hostname or ip address