def METHOD_NAME(ax):
    """Set the boundary of the given axis to be circular (for a polar plot)"""
    # Compute a circle in axes coordinates, which we can use as a boundary
    # for the map. We can pan/zoom as much as we like - the boundary will be
    # permanently circular.
    theta = numpy.linspace(0, 2*numpy.pi, 100)
    center = numpy.array([0.5, 0.5])
    radius = 0.5
    verts = numpy.vstack([numpy.sin(theta), numpy.cos(theta)]).T
    circle = matplotlib.path.Path(verts * radius + center)
    ax.set_boundary(circle, transform=ax.transAxes)