import pandas as pd
import matplotlib as mpl
import matplotlib.pyplot as plt
%matplotlib inline
data=pd.DataFrame({'Rating':['A','AAA','B','BBB','A','AAA','B','BBB'],
                  'Score':[2,4,5,6,2,4,5,6,]})
t=data.groupby('Rating', sort=False)['Score'].mean()
t
Rating
A      2
AAA    4
B      5
BBB    6
Name: Score, dtype: int64