Unexpected straight line when calling qchart real-time drawing in Ubuntu system

Phenomenon: I want to dawn a broken line to show the transfer speed. I use QChart chart class and QLineSeries real-time line, I take the transfer speed every 1 seconds and push the transfer value into QLineSeries.  On windows/Macosx/CentOS, it works as expected. But in Ubuntu, an unexpected straight line is drawed automatically.