import pandas as pd
df = pd.DataFrame({'group': ['a', 'a', 'a', 'a', 'a', 'a', 'a', 'b', 'b',
    'b', 'b', 'b', 'b', 'b'], 'day': ['Mon', 'Tues', 'Fri', 'Thurs', 'Sat',
    'Sun', 'Weds', 'Fri', 'Sun', 'Thurs', 'Sat', 'Weds', 'Mon', 'Tues'],
    'amount': [1, 2, 4, 2, 1, 1, 2, 4, 5, 3, 4, 2, 1, 3]})
df_grouped = df.groupby(['day']).sum().amount.reset_index()
weekdays = ['Mon', 'Tues', 'Weds', 'Thurs', 'Fri', 'Sat', 'Sun']
mapping = {day: i for i, day in enumerate(weekdays)}
key = df_grouped['day'].map(mapping)
df_grouped = df_grouped.iloc[key.argsort()]
df_grouped.plot(kind='bar', x='day')
Sorting the order of bars in pandas/matplotlib bar plots