Dense Qwt curves with dotted or dashed line style generally look solid.

[Uwe]

Curves with many points plotted with a Dotted or Dashed line style display as SOLID lines (not dotted or dashed) at normal zoom levels. We're using Qwt 6.1.3 and Qt 5.5.1 on Windows.

If your points are closer than the widths of the dashes/dots it is like this - that's how QPainter works.

So what you need is a curve interpolation, that weeds out points. In general QwtWeedingCurveFitter offers such an algo ( Douglas-Peucker ), but is an expensive one and should better be used in advance outside of the replot cycle, when having many points.

Uwe