The Aggregate Track field shows all top-level tasks in the trace file. The height of the graph indicates the number of concurrent tasks matching the aggregate criteria for the time slice corresponding to the horizontal position.
If you choose to display aggregate tracks, the tool displays the All Aggregate track, which combines all tasks across all threads in the trace. By default, the option Show Task Aggregates is selected.
You can add additional Aggregate tracks through the application menu. Check Displaying Aggregated Tasks with the Same Name.