The x axis may be drawn by xzeroaxis and removed by xzeroaxis. Similar commands behave similarly for the y, x2, y2, and z axes.
Syntax:
set {x|x2|y|y2|z}zeroaxis { {linestyle | ls <line_style>}
| { linetype | lt <line_type>}
{ linewidth | lw <line_width>}}
unset {x|x2|y|y2|z}zeroaxis
show {x|y|z}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).
zeroaxis is equivalent to yzeroaxis. Note that the z-axis must be set separately using zzeroaxis.
Examples:
To simply have the y=0 axis drawn visibly:
set xzeroaxis
If you want a thick line in a different color or pattern, instead:
set xzeroaxis linetype 3 linewidth 2.5