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switch variable: $colscale -- funct color scale
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switch value: 11 => blue/red to yellow/cyan

Two-condition statistic color scale for real
signed data.  Red is positive and blue is
negative correlation (can be reversed with
revphaseflag).  As significance increases, scale
goes red->yellow and blue->cyan (saturates more
slowly than BRw scale).  Not affected by
angle_offset, angle_cycles.

Unlike other scales, fmid should always be higher
than fthresh (fmid < fthresh gives green).  The
roll off to background is set by fthresh while
fmid sets the red->yellow (and blue->cyan)
transition points.
