functions in graph.i - b

             bytscl(z)  

          or bytscl(z, top=max_byte, cmin=lower_cutoff, cmax=upper_cutoff)  
 
     returns a char array of the same shape as Z, with values linearly  
     scaled to the range 0 to one less than the current palette size.  
     If MAX_BYTE is specified, the scaled values will run from 0 to  
     MAX_BYTE instead.  
     If LOWER_CUTOFF and/or UPPER_CUTOFF are specified, Z values outside  
     this range are mapped to the cutoff value; otherwise the linear  
     scaling maps the extreme values of Z to 0 and MAX_BYTE.  

builtin function, documented at i0/graph.i   line 1218