void yfgrid(lnstyle, nxdiv) GRIDOPTS.C Level 1
char lnstyle   = 0 for no y axis subdivisions.
 = -1 for tick marks only.
 = 19 selects the line style used by dashf().
int nydiv  The number of desired ticks between main grid lines on the y axis. For example, nxdiv = 9 will create 10 subdivisions. 
yfgrid() is like fgrid() except that grid lines are only drawn parallel to the x axis, intersecting the y axis. xfrgid() draws lines in the other direction.