QwtPlotCurve
http://qwt.sourceforge.net/class_qwt_plot_curve.html
inherits from QwtPlotSeriesItem
http://qwt.sourceforge.net/class_qwt_plot_series_item.html
There is a warning in setData
The item takes ownership of the data object, deleting it when its not used anymore.
It probably isn't the fact that you are growing too big... it may be that you are accessing something that Qwt deleted.
Run it in a debugger, and look at the stack trace for where it died, or put in a bunch of qDebug()
lines to see where it is dying.
If it is something with the data being too large, you could pop off items off of the head of your vector, before setting the vector.
Hope that helps.