Although I don't see it documented in the help, it seems to me that legends in plots are for CURVES or POINTS objects and not for POLYGONS.  To fix it, you could make an outline of the polygon at the same time as the polygon itself.

> outlinedpolygon := proc(L::list)
      uses plots;
      display(polygonplot(args),
        pointplot([op(L),L[1]], args[2..-1], style=line))
end proc;

> outlinedpolygon([[0,0],[1,1],[1,0],[0,0]],
     colour=blue,legend="stuff");

This looks like a bug in plots[polygonplot] which I'll record in our database. Until this is fixed, you can use the plottools[polygon] command as a workaround. This command takes similar arguments, but it creates a single POLYGONS structure rather than a plot. You'll have to call plots[display] to create the plot itself, but you can display several polygons at once. Here's an example:

> with(plottools);
> p1 := polygon([[1, 0], [2, 2], [1, 1]], color = "Blue", legend = "a triangle");
> p2 := polygon([[3, 3], [3, 2], [2, 3]], color = "Green", legend = "another triangle");
> plots[display](p1, p2);

Paulina Chin
Maplesoft