If you want to grab each data point, just using table with the fields _time and your field containing the memory info will do. Let's say the field is called memory_used:
... | table _time memory_used
After that, choose the chart view and apply the appropriate settings.
If you have loads of data points there is a risk of overwhelming the chart module with more points than it can handle. In that case, use timechart and some kind of statistical function for representing values in a certain time interval, like first, max or avg. You need to supply some kind of statistical function because timechart divides the events into discrete sets of time intervals, and it needs to know how to handle if there is more than 1 event in an interval.
... | timechart avg(memory_used)
