YZERO

Syntax: SET YZERO n
GET YZERO { n }
Default: YZERO = 0

YZERO controls whether or not to force zero to be displayed on the y-axis.

YZERO = 0 do not force zero to be displayed on the y-axis
YZERO ≠ 0 if YMIN ≥ 0 then set YMIN to zero, or, if YMAX ≤ 0, then set YMAX to zero.