def get_thresholdcurve_data(data, xname, yname):
    """
    Retrieves x and y columns from  of the data generated by the weka.classifiers.evaluation.ThresholdCurve
    class.

    :param data: the threshold curve data
    :type data: Instances
    :param xname: the name of the X column
    :type xname: str
    :param yname: the name of the Y column
    :type yname: str
    :return: tuple of x and y arrays
    :rtype: tuple
    """
    xi = data.attribute_by_name(xname).index
    yi = data.attribute_by_name(yname).index
    x = []
    y = []
    for i in range(data.num_instances):
        inst = data.get_instance(i)
        x.append(inst.get_value(xi))
        y.append(inst.get_value(yi))
    return x, y
