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.
Would it be better to leave out this `clabel` call, so we could see the labels specified directly below?