I am trying to eliminate the "lag" data before an arbitrary trigger event (only on the graph, but if the file need to be truncated so be it!) and am having no luck. Please see the graph below for reference.
Use a Combi Trigger module with the defined pre and post trigger settings, and then use it to control a Relay module where you route the rest of the data through. The Relay will release data while the trigger is active (high), and not while it's off (low).
You can write the data before the relay to write all of the data, or after the relay to only write the desired triggered area.