Plot a Horizontal line in Matplotlib

Matplotlib is a popular python library used for plotting, It provides an object-oriented API to render GUI plots

Plotting a horizontal line is fairly simple, Using axhline()

The axhline() function in pyplot module of matplotlib library is used to add a horizontal line across the axis.

Syntax: matplotlib.pyplot.axhline(y, color, xmin, xmax, linestyle)

Parameters:

• y: Position on Y axis to plot  the line, It accepts integers.
• xmin and xmax: scalar, optional, default: 0/1.  It plots the line in the given range
• color: color for the line, It accepts  a string. eg ‘r’ or ‘b’ .
• linestyle: Specifies the type of line, It accepts a string. eg ‘-‘, ‘–‘, ‘-.’, ‘:’, ‘None’, ‘ ‘, ”, ‘solid’, ‘dashed’, ‘dashdot’, ‘dotted’

Plotting a single horizontal line

Python3

# importing library
import matplotlib.pyplot as plt

# specifying horizontal line type
plt.axhline(y = 0.5, color = 'r', linestyle = '-')

# rendering the plot
plt.show()

Output:

Plotting multiple horizontal lines

To plot multiple horizontal lines, use the axhline() method multiple times.

Python

# importing the module
import matplotlib.pyplot as plt

# plotting line within the given range
plt.axhline(y = .5, xmin = 0.25, xmax = 0.9)

# line colour is blue
plt.axhline(y = 3, color = 'b', linestyle = ':')

# line colour is white
plt.axhline(y = 1, color = 'w', linestyle = '--')

# line colour is red
plt.axhline(y = 2, color = 'r', linestyle = 'dashed')

# adding axis labels
plt.xlabel('x - axis')
plt.ylabel('y - axis')

# displaying the plot
plt.show()

Output:

Adding the legend

The legend can be added using the legend() function.

Python3

# importing the module
import matplotlib.pyplot as plt

# plotting line within the given range
plt.axhline(y = .5, xmin = 0.25, xmax = 0.9)

# line colour is blue
plt.axhline(y = 3, color = 'b', linestyle = ':', label = "blue line")

# line colour is white
plt.axhline(y = 1, color = 'w', linestyle = '--', label = "white line")

# line colour is red
plt.axhline(y = 2, color = 'r', linestyle = 'dashed', label = "red line")

# adding axis labels
plt.xlabel('x - axis')
plt.ylabel('y - axis')

# plotting the legend
plt.legend(bbox_to_anchor = (1.0, 1), loc = 'upper center')

# displaying the plot
plt.show()

Output: