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
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