AnCo Hello!
It is a speed intervals chart from which a trip or track is estimated. This chart does not provide direct values of the speed transmitted by the tracker, but averaged over some specific intervals, based on the tracker data. In this case, you can see horizontal lines on the chart, which represent the average speed over a given interval. If one specific interval begins immediately after another specific interval, then end of first one and start point of another one coincide in time. You can see this in the places indicated in your screenshot.
If you want to see the speed graph directly based on the data transmitted by the tracker, then in the latest versions of the application there is a slide with such data and a separate chart.
