You're trying to add the same series more than once (that is, QSplineSeries * series_for_Vy has already been added to the chart). The fact that you changed the data in the series makes no difference.

You probably need to do this:

- remove the series from the chart (QChart::removeSeries())
- change the series data
- add the series back to the chart (QChart::addSeries())

You may not actually need to remove and add the series, but from what I remember of QChart, changing the data in the series may not result in an update to the chart.