How to shade and interpolate a scattered data file ?

["chi@xx" <chi@xxxxxxxxx>]

Dear ferret users
       I 'm writing this letter to ask for help about the "shade" 
command. I have an ascii point data file with around 1000 rows and each 
row contains three columns, "longitude", "depth" and "value".  I am 
looking for a way to shade the original point data without 
interpolation.  The commands I used are listed below
      yes? DEFINE AXIS/X=145:169:1 xax     ! the longitude range of my 
data is between 145 and 169
      yes? DEFINE AXIS/Y=-200:-4:1 yax      ! the depth range is 
between -200 and -4
      yes? SET DATA/EZ/VARIABLES="lats,lons,value"  mydata.txt
      After the 3 commands above, I don't know how to use "shade" 
command to draw the original point data. The basic syntax of shade is 
like        SHADE[/qualifiers] expression
      I have two questions here.
      1) Is "expression" here necessarily a variable or function upon a 
2D region? What should I do to shade "value" here? Do I need to define a 
2D variable for this purpose?
      2) I know I can use SCAT2GRIDGAUSS_XY  or SCAT2GRIDLAPLACE_XY to 
grid my scattered data.  Does each of these two external functions 
interpolate the point data on two dimensions? Since the longitude values 
of my data are not increased regularly, I don't want to interpoloate on 
longitude.  Can anybody show me how to just interpolate on one 
dimension, say depth?  
     Thanks a lot.  Any information will be highly appreciated.
     Ding Chi