The GPS provides the COG and the gyro-compass inside your phone provides the heading. The drift is the delta between. The actual value of the drift is less important, but what is really important to observe and improve the spread of this parameter. The better the boat (driver and trim) the less of the drift variation you will observe.
The black error indicates the mean of the distribution. The red error indicates the achievable target, which calculated based on our proprietary data mining algorithm.
Negative heel is heel to windward.