YGRID

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

YGRID controls the number of grid lines parallel to the x-axis. YGRID specifies the number of large, labeled, y-axis tic marks between each grid line.

YGRID meaning
> 0 draw a grid line parallel to the x-axis at every YGRID large tic mark, starting with the first.
= 0 suppress all grid lines parallel to the x-axis.
= -1 draw a grid line parallel to the x-axis at every y-axis tic mark, large and small. This option applies only if the axes are orthogonal.

See GRIDLINETYPE for information on changing the line type for the grid lines.