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' }
set noclabel
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.
See also set contour.