def fc_drift(data, n=3, **kwargs):
    """The 'drift' forecast of the next point in `data` (presumed to be 
    ordered in time) is a linear extrapoloation from `n` points ago, through the
    most recent point.
    
    `data` should be a 1-D numpy array
    `n` should be an integer
    
    Returns a single-valued forecast for the next value in the series.
    """
    yi = data[-n]
    yf = data[-1]
    slope = (yf - yi) / (n-1)
    forecast = yf + slope
    return forecast