Using QwtPlotRenderer to render a plot to a painter with high DPI (e.g. 1200) causes the legend icon to have incorrect line width. To reproduce, create a plot with a legend and curves that have the LegendShowLine attribute set. Set the pen for the curves to have a non-zero line width. Render the plot to a high DPI painter (e.g. using renderToDocument at 1200 dpi). The result is that the legend icons for those curves are drawn with a very small line width - small enough that you cannot see the dots in a dotted line.
This is with Qwt 6.1.0 (was not a problem with 6.0.1).
See the attached example program for an illustration of the problem.
From examining the code it seems that when the QwtGraphic is rendered, the transform of the painter is reset and the old transform applied to the path only (qwtExecCommand when doMap == true) so the scale that gets the line width into the correct dpi is lost. Removing the RenderPensUnscaled attribute in the legend icon stops that from happening so my current workaround is to subclass QwtPlotCurve, override the legendIcon method and set that attribute to false.