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tickbox var: $overlayflag -- toggle visibility stats
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keyboard equiv Linux: <Alt-o>
keyboard equiv Mac: <Cmd-o>

The "overlay" tickbox toggles visibility of
amplitude (val) data overlay.  It is autoset when
an overlay data set is read with the tkmedit
command line arguments:

  -real <format>
  -complex <cformat_r> <cformat_i>
