Stem Plot Using Python And Matplotlib

Overview:

  • Stem plot is a popular statistical tool that helps in exploratory data analysis.

 

  • A stem plot separates the digits in data points to form two columns.

 

  • Python Matplotlib draws a stem plot as a set of Y values plotted against common X-axis values.

 

  • The higher valued digit forms the left column – called stem.

 

  • The lower valued digit forms the values in the right column – called leafs.

 

  • The data is ordered in a stem plot.

 

  • The stems are from low value to higher values and so are the leafs.

 

  • The first leaf of the first stem forms the lowest value of the distribution.

 

  • The last leaf of the last stem forms the highest value of the distribution.

 

Why to use Stem Plots?

  • A stem plot helps visualizing the shape of the distribution.

 

  • Unlike the other graphic methods, stem plots retain their data value from the raw data to at least two digits.

 

How to construct a Stem plot manually:

  1. Creating stems: Order the data points. Plot only the left hand side digit of the data points and enter only once as a first column. Stems can be reused as well to form finer data ranges.
  2. Creating Leafs: On each data point the second digit should be placed one after another separated as commas as second column against the corresponding the stem.

Plotting a Stem Plot using Python:

  • The stem() function of the pyplot module is called with data for stems followed by the data for leafs i.e, X data followed by y data

Example:

import matplotlib.pyplot as plot

import numpy as np

import pylab

 

# marks obtained by students in an examination

marks = [10,11,22,24,35,37,45,47,48,58,56,59,61,71,81,92,95]

 

# corresponding stems

stems = [1,1,2,2,3,3,4,4,4,5,5,5,6,7,8,9,9]

 

# set the x axis and y axis limits

pylab.xlim([0,10])

pylab.ylim([0,100])

 

# Provide a title for the stem plot

plot.title('Stem Plot(Stem and Leaf Plot)')

 

# Give x axis label for the stem plot

plot.xlabel('Range/Stems')

 

# Give y axis label for the stem plot

plot.ylabel('Marks obtained/Leafs')

 

# plot the stem plot using matplotlib

markerline, stemlines, baseline = plot.stem(stems, marks, '-.')

 

# display the stem plot

plot.show()

 

Output:

Stem Plot or a Stem Leaf Plot

 


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