In matplotlib.pyplot, how to plot two datasets using interleaved bar graphs?

Question

I am going through Think Stats and I would like to compare multiple data sets visually. I can see from the book examples that it is possible to generate an interleaved bar g

Answer 1

Matplotlib's example code for interleaved bar charts works nicely for arbitrary real-valued x coordinates (as mentioned by @db42).

However, if your x coordinates are categorical values (like in the case of dictionaries in the linked question), the conversion from categorical x coordinates to real x coordinates is cumbersome and unnecessary.

You can plot two dictionaries side-by-side directly using matplotlib's api. The trick for plotting two bar charts with an offset to each other is to set align=edge and a positive width (+width) for plotting one bar chart, whereas a negative width (-width) for plotting the other one.

The example code modified for plotting two dictionaries looks like the following then:

"""
========
Barchart
========

A bar plot with errorbars and height labels on individual bars
"""
import matplotlib.pyplot as plt

# Uncomment the following line if you use ipython notebook
# %matplotlib inline

width = 0.35       # the width of the bars

men_means = {'G1': 20, 'G2': 35, 'G3': 30, 'G4': 35, 'G5': 27}
men_std = {'G1': 2, 'G2': 3, 'G3': 4, 'G4': 1, 'G5': 2}

rects1 = plt.bar(men_means.keys(), men_means.values(), -width, align='edge',
                yerr=men_std.values(), color='r', label='Men')

women_means = {'G1': 25, 'G2': 32, 'G3': 34, 'G4': 20, 'G5': 25}
women_std = {'G1': 3, 'G2': 5, 'G3': 2, 'G4': 3, 'G5': 3}

rects2 = plt.bar(women_means.keys(), women_means.values(), +width, align='edge',
                yerr=women_std.values(), color='y', label='Women')

# add some text for labels, title and axes ticks
plt.xlabel('Groups')
plt.ylabel('Scores')
plt.title('Scores by group and gender')
plt.legend()

def autolabel(rects):
    """
    Attach a text label above each bar displaying its height
    """
    for rect in rects:
        height = rect.get_height()
        plt.text(rect.get_x() + rect.get_width()/2., 1.05*height,
                '%d' % int(height),
                ha='center', va='bottom')

autolabel(rects1)
autolabel(rects2)

plt.show()

The result: