skip (expr, skipi, skipj)
 
Sets alternating values of expr to the missing
data value. 
expr     A valid grid expression that may have 1 or 2 varying dimensionsskipi    Skip factor in the I dimension of exprskipj    Skip factor in the J dimension of expr
arrows or barbs to thin the number of arrows or
barbs. It is not necessary to use the skip function on
both the U and V wind components; it is sufficient to populate only one
component with missing data values to suppress the plotting of the wind
arrow or barb.
set x 1
   set y 1
   set t 1 last
   d skip(var,2)d skip(u,2,2);v
    d skip(u,5,1);v
*
* This function does a d skip(ugrd,n);v
* where n is automatically set to an appropriate value
*
* usage: d_uv ugrd vgrd 
*
* v1.1 w. ebisuzaki
* v1.2 4/6/98 revised empirical formula for skip
*
function duv(arg)
u = subwrd(arg,1)
v = subwrd(arg,2)
* get lat/lon info
'query dims'
lons = sublin(result,2)
lats = sublin(result,3)
dx = subwrd(lons,13) - subwrd(lons,11)
dy = subwrd(lats,13) - subwrd(lats,11)
* Determine skip factor 
dn = dx
if (dy > dx) ; dn = dy ; endif
skip = dn / 50 + 0.5
if (skip < 1) ; skip=1 ; endif
* Display the plot
'd skip('u','skip');'v