What is the Matplotlib.pyplot.stem() method in Python?

Overview

Matplotlib is used to visualize statistical data for clear and concise understanding. It makes complicated data coherent with the aid of different graphs and plots.

Stem plot

A graph is used to classify data as leaf and step. It has a baseline with heads moving outwards away from the baseline.

Stem plot

The Matplotlib.pyplot.stem() method

The stem() method of the matplotlib.pyplot API is used to create a stem plot. It can be plotted in two ways either vertically or horizontally.

Horizontal stem plot

For the horizontal stem plot, locs are x positions while the heads are y values.

Vertical stem plot

For the vertical stem plot, locs are y positions while the heads are x values.

Syntax

matplotlib.pyplot.stem(locs, heads, linefmt=None, markerfmt=None, basefmt=None)

Parameters

  • locs: For the horizontal plot, these are y-positions of the stem. On the other hand, for a vertical plot, these value shows the x-position of the stem.
  • heads: For the horizontal plot, these are x-values of stem heads. On the other hand, for a vertical plot, these value shows the y-values of stem heads.
  • linefmt: This string value defines the color and style of vertical lines.
  • marketfmt: This string value defines the color and style of markers.
  • basefmt: This is a format for the baseline.

Return value

This method returns a tuple of marker lines, stemlines, and baselines.

Example

# import libraries in program
import matplotlib.pyplot as plt
import numpy as np
# generate x values
x = np.linspace(1, 2** np.pi)
# generate y values.
y = np.exp(np.cos(x))
# invoking stem() method
plt.stem(x, y, linefmt ='red', markerfmt ='-.', bottom = 1.1, use_line_collection = True)
# save above generated plot as png image in root directory
plt.savefig('output/graph.png')

Explanation

  • Lines 2 and 3: We import the matplotlib.pyplot API as plt and NumPy as np.
  • Line 5: We use the np.linspace() method that generates 50 sample ranges between 1 and 8.824977827076287.
  • Line 7: We calculate the exponent of np.cos(x) values and return a list of 50 samples, x.
  • Line 9: We pass the following parameters to the stem method:
    • locs and heads as a list of 50 values.
    • linefmt indicates lines will be red in color.
    • markerfmt shows the style of the marker.
    • use_line_collection is set to True, which means it will first use lines collection and then individual lines
  • Line 11: We use the plt.savefig() method to save the output graph as agraph.png file.

Free Resources

License: Creative Commons-Attribution NonCommercial-ShareAlike 4.0 (CC-BY-NC-SA 4.0)