Because the bad points are above and below the visible area. In dots style those are simply clipped away, in lines style you see the connections.
This sounds comprehensible, i gonna check this later this evening when i'm home in front of the code...

Some questions which came up this morning. Looking into the docs not clearly satisfied them:

1) How to make sure that QwtPlot contains only one curve? How check attached curves count?
2) Are there any methods to clear (detach) the curves of a plot in an efficient way?
3) What would be the best way to fully clear a curve before calling setRawSamples(..) in an updating routine?

Actually i call it within an iterative update routine like this (pseudo code):

Qt Code: 
  1. ...
  2. curve->setRawSamples(...);
  3. curve->attach(myPlot);
  4. myPlot->replot();
  5. ...
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thanks