How to Correctly Align Bars and Add Annotations in a Grouped Bar Chart with Matplotlib?

Plotting and Annotating a Grouped Bar Chart

This guide addresses a commonly encountered issue in matplotlib when creating a grouped bar chart. The provided code aims to visualize respondents' interest in various data science areas with bars representing their levels of interest (Very interested, Somewhat interested, Not interested).

Problem Analysis

The issue in the provided code lies in the calculation of bar widths. The code sets w=0.8 without considering the number of bars in the plot. To align the bars correctly, w should be divided by the number of bars.

Solution

To resolve this issue, adjust the bar width calculation to consider the number of bars in the plot. A more efficient approach is to use the pandas.DataFrame.plot method to generate the plot with annotations.

Updated Code Using DataFrame.plot

<code class="python">import pandas as pd
import matplotlib.pyplot as plt

# Create the DataFrame
file = "https://s3-api.us-geo.objectstorage.softlayer.net/cf-courses-data/CognitiveClass/DV0101EN/labs/coursera/Topic_Survey_Assignment.csv"
df = pd.read_csv(file, index_col=0)

# Calculate percentages
df = df.div(2233)

# Plot the grouped bar chart
ax = df.plot.bar(color=['#5cb85c', '#5bc0de', '#d9534f'], figsize=(20, 8), rot=0, ylabel='Percentage', title="The percentage of the respondents' interest in the different data science Area")

# Add annotations
for p in ax.patches:
    ax.annotate(f'{p.get_height():0.2f}', (p.get_x() + p.get_width() / 2., p.get_height()), ha='center', va='center', xytext=(0, 10), textcoords='offset points')</code>

Output:

[Image of the corrected bar chart]

This updated code generates a properly aligned grouped bar chart with annotations indicating the percentages for each bar.

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