We're running into an issue where lines are not drawn the same between the plot and the legend. The following screenshot shows the difference:

Here's a closer look at the lines (Both colors were changed to black):
The lines as they appear in the legend and the plot are on the left and right, respectively. The lines in the top row both have a specified line width of 1, while the lines in the bottom row both have a specified line with of 2.

We are seeing this in Qwt 6.1.4 and Qt 5.12.4.

Is there some setting or treatment we need to apply to make the plot and legend match more closely?