def get_center_and_radius(_, ax):
    """Return center, radius, both are (x, y) tuples
    Param: event, ax
    """
    xlim = ax.get_xlim()
    ylim = ax.get_ylim()
    rx = (xlim[1] - xlim[0]) * 0.5
    ry = (ylim[1] - ylim[0]) * 0.5
    cx = xlim[0] + rx
    cy = ylim[0] + ry
    return (cx, cy), (rx, ry)
