def click_estimate_curvature():
    """
    Takes two clicks and returns the curvature, assuming the first click
    was the minimum of a parabola and the second was some other point.

    Returns the second derivative of the function giving this parabola.

    Right-click aborts.
    """

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

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

    return 2*(c2[0][1]-c1[0][1])/(c2[0][0]-c1[0][0])**2