Syntax:
set zeroaxis {<linestyle>} set xzeroaxis {<linestyle>} set yzeroaxis {<linestyle>} set x2zeroaxis {<linestyle>} set y2zeroaxis {<linestyle>} set nozeroaxis set noxzeroaxis etc. show zeroaxis show xzeroaxis etc.
By default, these options are off. If <linestyle> is not specified, any zero axes selected will be drawn using the axis linetype.
set zeroaxis l is equivalent to set xzeroaxis l; set yzeroaxis l. set nozeroaxis is equivalent to set noxzeroaxis; set noyzeroaxis.