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Plot a Horizontal line in Matplotlib

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  • Last Updated : 12 Nov, 2020
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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:


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