def plane(xy_array, a, b, c):
    """
    Return z values such that z = a*x + b*y + c.

    :param xy_array: a (2xN) numpy array, x values being the first row and
     y values the second row
    :param a: coefficient for x values
    :param b:  coefficient for y values
    :param c: constant offset
    :return: an array of length N containing the z values
    """
    return a * xy_array[0, :] + b * xy_array[1, :] + c