Using matplotlib 0.74.
The default linestyle for contour lines representing values less than zero should be dashed.
I have used this in the past ( I guess prior to 0.74) but now the behavior is different.
The default appears to be a greyscale color map - all solid lines.
Looking at the code - line 768 of contour.py, the negative lines are dashed if Ncolors == 1.
As far as I can tell, there is no way for this to be true. The code change to rectify this is trivial,
but I may be missing something.
I could easily be doing something really dumb - but it seems pretty straightforward.
Thanks for any help.