def log_scale(N=8, start=-9, end=3, round_to=3):
    """creates an array of numbers on logarithmic scale with:
        - number per decade
        - start decade
        - end decade
        - round_to number of digits after decimal, default is 3
        """
    from numpy import logspace, around
    arr = around(logspace(start, end, (end - start) * N + 1, endpoint=True),
        abs(start - round_to))
    return arr
