http://maxima.sourceforge.net/docs/manual/en/maxima_48.html
Graphic option: transparent
Default value: false
If transparent is true, interior regions of polygons are filled according to fill_color.
This option affects the following graphic objects:
gr2d: polygon, rectangle, and ellipse.
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*** This is backwards; if transparent is _false_, interior regions of polygons are filled according to fill_color. ***