Overlapping Group Barcharts in Matplotlib

In this article, we will delve into the world of group barcharts and explore a common issue that arises when plotting overlapping bars using matplotlib. We’ll examine the cause of the problem, understand how to avoid it, and provide a step-by-step guide on how to create non-overlapping barcharts.

What are Group Barcharts?

A group barchart is a type of bar chart where multiple bars share the same x-axis values but have different y-values. This is often used to compare data across categories or groups. In matplotlib, we can create a group barchart using the bar function with the align='edge' argument.

The Problem: Overlapping Bars

When creating a group barchart with overlapping bars, the problem arises when the bars are too close together. This causes the bars to appear as if they are not aligned properly on the x-axis. In our example from the Stack Overflow post, we can see that the green and red bar charts are overlapping, which makes it difficult to read the data.

Why Does This Happen?

There are several reasons why overlapping bars occur in a group barchart:

• Insufficient space between bars: When there is not enough space between the bars, they tend to overlap.
• Incorrect alignment: If the bars are not aligned properly on the x-axis, it can cause them to appear as if they are overlapping.

How to Create Non-Overlapping Barcharts

To avoid overlapping bars in a group barchart, you can use the following techniques:

1. Increase the Space Between Bars

You can increase the space between bars by using the bar_width argument when creating the plot. This will give each bar more room to breathe and prevent them from overlapping.

fig, ax = plt.subplots()
index = np.arange(n_groups)
bar_width = 0.4  # Increase the bar width

rects1 = ax.bar(index - bar_width/2, GreenTaxi, bar_width,
               alpha=opacity,
               color='b',
               label='GreenTaxi')

rects2 = ax.bar(index + bar_width/2, YellowTaxi, bar_width,
               alpha=opacity,
               color='g',
               label='YellowTaxi')

2. Use a Different Alignment

You can also use the align argument to specify how the bars should be aligned on the x-axis. By default, matplotlib aligns the bars at the center of each tick mark. However, you can change this by using the edge alignment.

fig, ax = plt.subplots()
index = np.arange(n_groups)
bar_width = 0.35

rects1 = ax.bar(index - bar_width/2, GreenTaxi, bar_width,
               alpha=opacity,
               color='b',
               label='GreenTaxi')

rects2 = ax.bar(index + bar_width/2, YellowTaxi, bar_width,
               alpha=opacity,
               color='g',
               label='YellowTaxi')

3. Use a Different Plotting Function

In some cases, you may need to use a different plotting function to create your group barchart. For example, you can use the barh function to create a horizontal bar chart.

fig, ax = plt.subplots()
index = np.arange(n_groups)
bar_width = 0.35

rects1 = ax.barh(index - bar_width/2, GreenTaxi, bar_width,
                alpha=opacity,
                color='b',
                label='GreenTaxi')

rects2 = ax.barh(index + bar_width/2, YellowTaxi, bar_width,
                alpha=opacity,
                color='g',
                label='YellowTaxi')

Example Use Case

Let’s create a group barchart using the techniques we discussed earlier. We’ll plot three different datasets: Uber, GreenTaxi, and YellowTaxi.

import numpy as np
import matplotlib.pyplot as plt

# Data to plot
n_groups = 6
GreenTaxi = [10, 15, 7, 12, 20, 18]
YellowTaxi = [8, 14, 5, 11, 19, 17]
Uber = [9, 13, 6, 10, 21, 16]

# Create plot
fig, ax = plt.subplots()
index = np.arange(n_groups)
bar_width = 0.4

rects1 = ax.bar(index - bar_width/2, GreenTaxi, bar_width,
               alpha=opacity,
               color='b',
               label='GreenTaxi')

rects2 = ax.bar(index + bar_width/2, YellowTaxi, bar_width,
                alpha=opacity,
                color='g',
                label='YellowTaxi')

rects3 = ax.bar(index + bar_width, Uber, bar_width,
                alpha=opacity,
                color='r',
                label='Uber')

ax.set_xlabel('Month')
ax.set_ylabel('Ride Counts')
ax.set_title('Rides By Month')
plt.xticks(index + bar_width/2, ['Jan', 'Feb', 'March', 'April', 'May', 'June'])
ax.legend()
plt.show()

In this example, we’ve increased the space between bars by using a larger bar_width and changed the alignment to edge. This has resulted in a non-overlapping group barchart with clear labels.

Conclusion

In conclusion, creating a group barchart with overlapping bars can be frustrating. However, by increasing the space between bars, using a different alignment, or using a different plotting function, you can avoid overlapping bars and create a clear and readable chart. By following the techniques we discussed in this article, you should be able to create high-quality group barcharts that accurately represent your data.

Last modified on 2023-06-07