15.40.14  DX 

DX|DY|DZ  are  the  separation  between  parallel  lines used to shade the
faces.  If zero parallel lines are not drawn.

                                  Example
You  wish  to shade face YZ,ZX with lines parallel to the Y axis separated
by 0.1 units.  Essentially this draws  lines  at  even  multiples  of  0.1
units.  This may be used in a similar fashion to grid lines on 1-d plots.
     TD:HISTOGRAM YZ ZX DX=0.1
Note:   you  specify  DX the distance along the X axis.  Since DY is 0, no
lines parallel to the X axis are drawn.  When selecting RANDOM shading  DX
specifies the number of dots/face.