Syntax:
set {x|x2|y|y2|z}zeroaxis { {linestyle | ls <line_style>} | {linetype | lt <line_type>} {linewidth | lw <line_width>} {linecolor | lc <colorspec>} {dashtype | dt <dashtype>} } 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 4#4line_type5#5, width 4#4line_width5#5, color 4#4colorspec5#5, and dash type 4#4dashtype5#5 (if supported by the terminal driver currently in use), or a user-defined style 4#4line_style5#5 (see set style line (p. )).
If no linetype is specified, any zero axes selected will be drawn using the axis linetype (linetype 0).
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