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