def click_estimate_difference():
    """
    Takes two clicks and returns the difference vector [dx, dy].

    Right-click aborts.
    """

    c1 = _pylab.ginput()
    if len(c1)==0:
        return None

    c2 = _pylab.ginput()
    if len(c2)==0:
        return None

    return [c2[0][0]-c1[0][0], c2[0][1]-c1[0][1]]
Takes two clicks and returns the difference vector [dx, dy].

    Right-click aborts.