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tickbox variable: $zerosymmfadeflag -- for color LUT
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default: 0 (FALSE)

If the "sym" tickbox (upper right) is ticked,
(s)fthresh and (s)fmid cut into the color scale
symmetrically in both directions from zero.

If not ticked, thresholds only cut in from the
bottom.

See "intp" tick to control whether colors in
between table entries are interpolated or
constant, and "bot" tick to control whether entry
sets bottom edge or center of color,

See help for "lut:" entry for more details.

N.B.: no effect on phase LUT (assumed OFF).
