legend handle size does not automatically scale with linewidth

If I use a line style that's dash-dot plus a linewidth that's greater than 1, the linestyle doesn't appear in the legend if numpoints > 1.

Here's linewidth == 1 and various line styles:

And here's linewidth == 2 and various line styles:

Legend is drawn with

leftaxis.legend(handles=lines, loc='best', numpoints=3, frameon=True, fancybox=True, framealpha=0.5)

where lines are generated with Axes.plot.

Matplotlib version

Linux 16.04
Matplotlib 2.0.2 with Agg backend, from source
Python 2.7.13

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