The x axis may be drawn by set xzeroaxis and removed by set noxzeroaxis. Similar commands behave similarly for the y, x2, and y2 axes.
Syntax:
set {x|x2|y|y2|}zeroaxis { {linestyle | ls <line_style>}
| { linetype | lt <line_type>}
{ linewidth | lw <line_width>}}
set no{x|x2|y|y2|}zeroaxis
show {x|y|}zeroaxis
By default, these options are off. The selected zero axis is drawn with a line of type <line_type> and width <line_width> (if supported by the terminal driver currently in use), or a user-defined style <line_style>.
If no linetype is specified, any zero axes selected will be drawn using the axis linetype (linetype 0).
set zeroaxis l is equivalent to set xzeroaxis l; set yzeroaxis l. set nozeroaxis is equivalent to set noxzeroaxis; set noyzeroaxis.