Dense curves don't show dotted or dashed line style; they look solid.

Question posted to the QtCentre / Qwt forum. 4-05-2017.
Dense Qwt curves with dotted or dashed line style generally look solid.
http://www.qtcentre.org/threads/68094

Note [4-20-2017]: This problem has been addressed for RiverWare 7.1. This is described on this webpage:

Gnats 5953: Plotting: Patterned lines sometimes look solid, esp. with thickness greater than one.
http://cadswes2.colorado.edu/~philw/pub/5953/DensePattCurveFix.html

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.

One thing I thought to try was turning ON the "FilterPoints" PaintAttribute on the QwtPlotCurves. But I found that these curves already had it on (by default). (And turning it off didn't fix the problem).

The images below (expanded X2) show one "Step" curve and one "Line" curve with many points. This sequence of seven images progressively zooms in. Point symbols are turned on half-way through to show the density of the series data.

Is there any way of having the Dotted and Dashed line styles work for dense curve data?

Animated sequence of seven frames:

The individual frames:

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