Millisecond Forums

Hi,
We have a question about eye-tracking output we generated with the Inquisit Tobii plugin. The output file lists x,y,z coordinates of the gaze points of the eyes and coordinates of the eyes. Millisecond documentation states that both are reported 'in the 3D User Coordinate System'. Tobii SDK states this is in millimeters, as unit. Attached a recording plot of both variables (example from left-eye z-dimension). Unclear to us are the units and origin of the gaze data (y axis) in particular.  
Thanks for any clarification,
Esther