How force certain plot items be ON TOP and others be COVERED?

This is what the zorder is used for. Artists should have

    > a zorder attribute. The higher the zorder, the more in
    > front an object is. I think they all default to 1.

Different types of "Artists" have different default zorders

  In [2]: from matplotlib.patches import Patch

  In [3]: from matplotlib.lines import Line2D

  In [4]: Line2D.zorder
  Out[4]: 2

  In [5]: Patch.zorder
  Out[5]: 1

So by default lines appear above patches....

See also examples/