import pandas as pd

# Calculate review frequencies by year
data['year'] = pd.DatetimeIndex(data['period']).year
rows_count = data.groupby('year', as_index=False).year.value_counts()
rows_count.columns=['year','reviews']

import matplotlib.pyplot as plt

# Generate a line plot
rows_count.plot.line(x='year',y = 'reviews')
plt.show()