การแปลงข้อมูลดิบ

# Your custom function
def percent_change(series):
    # Collect all *but* the last value of this window, then the final value
    previous_values = series[:____]
    last_value = series[-1]

    # Calculate the % difference between the last value and the mean of earlier values
    percent_change = (____ - np.mean(previous_values)) / np.mean(previous_values)
    return percent_change

# Apply your custom function and plot
prices_perc = prices.rolling(20).____
prices_perc.loc["2014":"2015"].plot()
plt.show()