The trace is the resulting graph of the variable's evolution against time, with the more distant past on the left and the more recent past on the right.
The acquisition of samples is limited to 100'000 cycles (ie. 100 s when cycle time is set to 1000 µs, and 25 s when cycle time is set to 250 µs). When you reach this limit:
The first data that are added to the queue are the first data to be removed (FIFO queue)