The set zlabel command sets the z-axis label that is centered along the z axis. Using the optional x,y screen offsets, the label can be placed anywhere on the plot. set zlabel with no parameters clears the label.
Syntax:
set zlabel {"<label>"} {<xoff>}{,<yoff>}
show zlabel
Specifying constants <xoff> or <yoff> as optional offsets for the label will move the label <xoff> or <yoff> character screen coordinates. For example,
set zlabel ,1
will change only the y offset of the zlabel, moving the label roughly one character height up.
The zlabel will be drawn whenever surfaces or contours are plotted, in the space above the grid level.
(The EEPIC, Imagen, LaTeX, and TPIC drivers allow 
in a string to specify
a newline.)