This patch adds the possibility to manage discontinuity (or gaps) in QwtPlotCurve.
For this, it just needed to add two virtual members to QwtSeriesData :
-nDiscontinuities() which returns the number of discontinuities
-discontinuity(size_t i) which returns the data index for the ith discontinuity
Obviously, the default implementation of nDiscontinuities returns zero, so all existing code are compatible with this patch
Currently it just works for lines, but it would be a very easy thing to add this feature to Steps, or whatever kind of QwtPlotCurve.
The advantage over existing patch (like http://www.qtcentre.org/threads/21756-How-to-create-QWT-plot-curve-with-gaps) is that it does not imply to have a special data value for discontinuities, and it does not add any data reading loop, so there is nearly no impact on performances.
I'm completely open remarks, and ready to improve this feature, so that it can make its way inside qwt