Get transverse coord at highest z

        :param zx: A list where each element is (z, transverse, charge)

def _get_last_transverse_over_list(self, zxq):
        """ Get transverse coord at highest z

        :param zx: A list where each element is (z, transverse, charge)
        """
        z_max = None
        x_of_interest = None

        for z, x, q in zxq:
            if z == None or z > z_max:
                x_of_interest = x

        return x_of_interest