Bug 5864: Inconsistent appearance of lines in Plots and Legend with thickness of 2 or more.
Bug Number: 5864
Release notes (y/n): Yes (if not put into 7.0.x)
For Release Nums: 7.1, maybe 7.0.7
Gnats 5864: Legend and plot line not the same if thickness is greater than 1.

The problem, and the effect of the fix, are shown on this webpage:
http://cadswes2.colorado.edu/~philw/2017/bugs/5864/LegendProb.html

The fix is modifying this Qwt virtual method: QwtPlotCurve::legendIcon(), 
by lifting that code, and adapting it for SlotCurve::legendIcon().  
Commenting out a call to QPen::setCapStyle (Qt::FlatCap) fixes the problem.

I posted a question about this on the QtCentre / Qwt forum:
http://www.qtcentre.org/threads/68045  

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