def monotonic(values, mode="<", atol=1.0e-8):
    """
    Returns False if values are not monotonic (decreasing|increasing).
    mode is "<" for a decreasing sequence, ">" for an increasing sequence.
    Two numbers are considered equal if they differ less that atol.

    .. warning:
        Not very efficient for large data sets.

    >>> values = [1.2, 1.3, 1.4]
    >>> monotonic(values, mode="<")
    False
    >>> monotonic(values, mode=">")
    True
    """
    if len(values) == 1:
        return True

    if mode == ">":
        for i in range(len(values) - 1):
            v, vp = values[i], values[i + 1]
            if abs(vp - v) > atol and vp <= v:
                return False

    elif mode == "<":
        for i in range(len(values) - 1):
            v, vp = values[i], values[i + 1]
            if abs(vp - v) > atol and vp >= v:
                return False

    raise ValueError("Wrong mode %s" % str(mode))