Thanks, KiriM.

I had to implement some tweaks to get this to work in qwt-6.0.0 because the draw() function is different in 6.0.0.

I overrode QwtPlotCurve::drawSeries(), included the QRectF of the canvas in it's constructor, and used the d_deries->samples(i).y() data-values in the comparison to "gapValue_"
Qt Code: 
  1. ////////////////////////////////////////////////////////////////////////////////
  2. void QwtPlotGappedCurve::drawSeries(QPainter *painter, const QwtScaleMap &xMap,
  3.                                                           const QwtScaleMap &yMap, const QRectF &canvRect, int from, int to) const
  4. {
  5. 	if ( !painter || dataSize() <= 0 )
  6. 		return;
  7.  
  8. 	if (to < 0)
  9. 		to = dataSize() - 1;
  10.  
  11. 	int i = from;
  12. 	while (i < to)
  13. 	{
  14.         // If data begins with missed values,
  15.         // we need to find first non-missed point.
  16.             double y = d_series->sample(i).y();
  17.             while ((i < to) && (y == gapValue_))
  18.             {
  19.                 ++i;
  20.                 y = d_series->sample(i).y();
  21.             }
  22.         // First non-missed point will be the start of curve section.
  23.             int start = i;
  24.             y = d_series->sample(i).y();
  25.         // Find the last non-missed point, it will be the end of curve section.
  26.             while ((i < to) && (y != gapValue_))
  27.             {
  28.                 ++i;
  29.                 y = d_series->sample(i).y();
  30.             }
  31.         // Correct the end of the section if it is at missed point
  32.             int end = (y == gapValue_) ? i - 1 : i;
  33.  
  34.         // Draw the curve section
  35.             if (start <= end)
  36.                     QwtPlotCurve::drawSeries(painter, xMap, yMap, canvRect, start, end);
  37. 	}
  38. }
  39.  
  40. ////////////////////////////////////////////////////////////////////////////////
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