I'd like to chart free memory vs. used memory over time on the same Splunk dashboard module. I'm trying to figure out a good visualization to use.
This sounds like a pretty common charting use-case. Splunk experts: how have you visualized free vs. used metrics in your own reports, and what Splunk commands did you use to create that visualization?
I typically use a stacked area or column chart to represent free vs. used quantities. This is because together the free+used should equal the total in the system. Of course, in some metrics, the OS itself will take a slice that's either unreported or reported as buffers/cached. If you want to preserve the relative usage regardless of system usage, you can switch from an ordinary stacked chart to a 100% stacked chart.
Assuming that you have two fields in your time-series data, one for free and one for used, you can just use timechart: ... | timechart median(free) median(used). If you want to look at extremal behavior, use min(free) and max(used).