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tickbox variable: $complexvalflag -- toggle complex data
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default: 0 (FALSE)

The "cpx" tickbox (just right of center) toggles
whether data is displayed as real-valued or
complex-valued.

When "cpx" is ticked ($complexvalflag is 1), the
functional data value is assumed to have two
components (real, imaginary).  A phase angle is
calculated and used to generate the color scale.
The colorscale radio buttons reset this flag
appropriately.

N.B.: if a complex-valued colscale (0,1,4,8,18)
is currently set and "cpx" is unticked, the
colscale is changed to real-valued
(colscale=6=BR) to avoid showing a blank surface.

N.B.: If a real-valued colscale (6,11) is
currently set and "cpx" is ticked, the colscale
is changed to complex-valued (colscale=0=w1) to
avoid showing a blank surface.

N.B.: complex data displayed using a real-valued
color scale or real data displayed using a
complex-valued color scale can be confusing to
interpret.

N.B.: colscales 15 and 17 can display real-valued
data or the amplitude of complex-valued data.

