15.15.1  N1...  

Contour lines are drawn for the specified values n1,n2...

If  no  values  are  specified, then lines are drawn according to the tick
locations selected by the SET SCALE command.  Primary  contour  lines  are
determined  by  the  long  tick  locations, and the secondary lines by the
short ticks.

If the scale of the Z axis is set to  be  logarithmic,  then  the  primary
lines are at powers of 10.