15.59.5  X|Y|Z 

This  specifies the axis to project along.  If you specify v then only the
row/column corresponding to v is projected.  If specified v1  and  v2  may
select  a range of values to project.  If no values are specified then the
entire mesh is  projected.   If  BY=v3  is  specified  then  a  series  of
projections  are  made  from  v1  to  v2  with  a  width of v3.  If N=n is
specified then the range from v1 to v2 is split into  n  equal  intervals.
You  need  not  use  this  command  if you wish to histogram only a single
slice.  The SLICE modifier on the HISTOGRAM, or JOIN commands may be used.
If  X,Y,  or  Z  are  repeated  then  extra  data  sets  are  defined with
projections.