How to interpret the free memory figures in the Health graphs

[eponymous]

Hi,

I'm trying to track memory usage and looked at the health graph for memory.

I've removed all the lines except "free" but I can't figure out how to interpret the values.

My memory is ranging from "28" to about "3.2 / 3.3" but what does this actually mean? Looking at the dashboard shows about 68% usage around the time it's reporting 3.2 so I just assumed this was some sort of percentage i.e. 100-68 = 32. But then what does "28" mean?

I'm sure this is something simple I'm missing!

Thanks.