psychemedia psychemedia - 1 month ago 8
Python Question

Sorting the order of bars in pandas/matplotlib bar plots

What is the Pythonic/pandas way of sorting 'levels' within a column in pandas to give a specific ordering of bars in bar plot.

For example, given:

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]})
dfx=df.groupby(['group'])
dfx.plot(kind='bar',x='day')


I can generate the following pair of plots:

Disordered bar charts

The order of the bars follows the row order.

What's the best way of reordering the data so that the bar charts have bars ordered Mon-Sun?

UPDATE: this rubbish solution works - but it's far from elegant in the way it uses an extra sorting column:

df2=pd.DataFrame({'day':['Mon', 'Tues', 'Weds','Thurs','Fri','Sat','Sun'],'num':[0,1,2,3,4,5,6]})
df=pd.merge(df,df2,on='day')
df = df.sort('num')
dfx=df.groupby(['group'])
dfx.plot(kind='bar',x='day')


FURTHER GENERALISATION:

Is there a solution that also fixes the order of bars in a 'dodged' bar plot:

df.pivot('day','group','amount').plot(kind='bar')


enter image description here

Answer Source

You'll have to provide a mapping to specify how to order the day names. (If they were stored as proper dates, there would be other ways to do this.)

Updated:

Build the key. You could write out a dictionary explicitly or use something clever like this dict comprehension.

weekdays = ['Mon', 'Tues', 'Weds', 'Thurs', 'Fri', 'Sat', 'Sun']
mapping = {day: i for i, day in enumerate(weekdays)}
key = df['day'].map(mapping)

And the sorting is simple:

df.iloc[key.argsort()]