set clab option
Controls contour labeling. The
option argument may be one of the following:
on'fast' contour labels are plotted where the contour lines are horizontal
offno contour labels
forcedan attempt is made to label all contour lines
formatgives a C-language template for conversion of the contour value to a string
masked(version 2.0.a6+) contour lines have gaps for the labels, so rectangles for label background are not drawn; contour labels never overlap.