BUGFIX:  This change fixes #2475, where contour labels added manually

produce ugly spikes in the contours.  The problem was that screen
coordinates were being passed as data coordinates when the contour was
split to add room for the label.

Incorporate @bwkeller fix and @miili's #2806 change

See long comment on #2818 for why this is the correct fix.
I think this comment is not helping; the nearby docstring and the code are readable as they are.