def poly_below(ymin, xs, ys):
return poly_between(xs, ys, xmin)<EOL>
given a arrays *xs* and *ys*, return the vertices of a polygon
that has a scalar lower bound *ymin* and an upper bound at the *ys*.

intended for use with Axes.fill, eg::

  xv, yv = poly_below(0, x, y)
  ax.fill(xv, yv)