Returns a default spacing between consecutive ticks for annual data.

def _get_default_annual_spacing(nyears):
    """
    Returns a default spacing between consecutive ticks for annual data.
    """
    if nyears < 11:
        (min_spacing, maj_spacing) = (1, 1)
    elif nyears < 20:
        (min_spacing, maj_spacing) = (1, 2)
    elif nyears < 50:
        (min_spacing, maj_spacing) = (1, 5)
    elif nyears < 100:
        (min_spacing, maj_spacing) = (5, 10)
    elif nyears < 200:
        (min_spacing, maj_spacing) = (5, 25)
    elif nyears < 600:
        (min_spacing, maj_spacing) = (10, 50)
    else:
        factor = nyears // 1000 + 1
        (min_spacing, maj_spacing) = (factor * 20, factor * 100)
    return (min_spacing, maj_spacing)