How to Create Error Bar Plot in Matplotlib: Simple Guide

Use the matplotlib.pyplot.errorbar() function to create error bar plots by passing your data points and their corresponding error values. Specify x, y, and yerr (or xerr) to show vertical or horizontal error bars respectively.

Syntax

The basic syntax of errorbar() is:

• x: x-coordinates of data points.
• y: y-coordinates of data points.
• yerr: vertical error values (can be scalar, array, or list).
• xerr: horizontal error values (optional).
• fmt: format string for data points (e.g., 'o' for circles).
• ecolor: color of error bars.

This function plots data points with error bars showing uncertainty or variability.

python

matplotlib.pyplot.errorbar(x, y, yerr=None, xerr=None, fmt='', ecolor=None, capsize=None, capthick=None, elinewidth=None, errorevery=1, barsabove=False, lolims=False, uplims=False, xlolims=False, xuplims=False, **kwargs)

Example

This example shows how to plot points with vertical error bars using yerr. It demonstrates data points with symmetric error bars.

python

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(0, 10, 10)
y = np.sin(x)
error = 0.2

plt.errorbar(x, y, yerr=error, fmt='o', ecolor='red', capsize=5)
plt.title('Error Bar Plot Example')
plt.xlabel('X axis')
plt.ylabel('Y axis')
plt.show()

Output

A plot window showing 10 data points as circles along a sine curve with red vertical error bars of length 0.2 and caps at the ends.

Common Pitfalls

Common mistakes when creating error bar plots include:

• Not matching the length of yerr or xerr arrays to the data points length.
• Using fmt as an empty string which hides data points.
• Forgetting to set capsize to make error bar caps visible.
• Passing error values as negative numbers, which is invalid.

Always ensure error arrays are positive and correctly sized.

python

import matplotlib.pyplot as plt
import numpy as np

x = np.arange(5)
y = np.random.rand(5)

# Wrong: yerr length mismatch
# plt.errorbar(x, y, yerr=[0.1, 0.2], fmt='o')  # This will raise an error

# Right:
plt.errorbar(x, y, yerr=0.1, fmt='o', capsize=3)
plt.show()

Output

A plot window showing 5 data points with uniform vertical error bars of length 0.1 and caps.

Quick Reference

Parameter	Description	Example
x	X coordinates of data points	[1, 2, 3, 4]
y	Y coordinates of data points	[2, 3, 5, 7]
yerr	Vertical error bar sizes	0.2 or [0.1, 0.2, 0.3, 0.4]
xerr	Horizontal error bar sizes	0.1 or [0.05, 0.1, 0.15, 0.2]
fmt	Format of data points	'o' for circles, '-' for line
ecolor	Color of error bars	'red', 'blue'
capsize	Length of error bar caps	3 or 5

Key Takeaways

Use matplotlib.pyplot.errorbar() with x, y, and yerr to create error bar plots.

Ensure error arrays match data length and contain positive values.

Set fmt to show data points and capsize to make error bar caps visible.

You can add horizontal error bars using xerr parameter.

Customize error bar color with ecolor for better visualization.