If lower and high values are equal ensures they are separated by
    the defined span.

def get_minimum_span(low, high, span):
    """
    If lower and high values are equal ensures they are separated by
    the defined span.
    """
    if is_number(low) and low == high:
        if isinstance(low, np.datetime64):
            span = span * np.timedelta64(1, 's')
        low, high = low-span, high+span
    return low, high