Thanks for your library.
However there seems to be a bug in QwtPlotCurve in version libqwt 6. It executes 'delete symbol' internally for each symbol when the plot is destroyed. This is a problem when one has more than one symbol allocated together in a chunk (or through static memory), in which case each symbol must be allocated separately.
curve = new QwtPlotCurve*[maxN];
symbol = new QwtSymbol[maxN};
curve[i]= new QwtPlotCurve();