Problem

if you contour a negative quantity, it gets dashed lines. Leaving aside whether this is a good default or not, the only way to toggle this is via rcParams['contour.negative_linestyle']=False.

Proposed solution

I think this should be togglable via kwarg, though I appreciate that overlaps with linestyle and only is activated with monochrome contours.

(I actually think the default should be False, FWIW - this surprises me every time, and I make quite a few contour plots).