`gnuplot` will vary the linetype used for each contour level when clabel is set. When this option on (the default), a legend labels each linestyle with the z level it represents. It is not possible at present to separate the contour labels from the surface key.
Syntax:
set clabel {'<format>'}
unset clabel
show clabel
The default for the format string is %8.3g, which gives three decimal places. This may produce poor label alignment if the key is altered from its default configuration.
The first contour linetype, or only contour linetype when clabel is off, is the surface linetype +1; contour points are the same style as surface points.
See also contour.