Drawing A Violin Plot Using Python And Matplotlib

Overview:

  • A violin plot combines two aspects of a distribution in a single visualization:
    • The features of a Box Plot: Median, Interquartile Distance
    • The Probability Density Function
  • In a violin plot, the Probability Density Function-PDF of the distribution is tilted side wards and placed on both the sides of the box plot.
  • A violin plot clearly displays the multiple modes present in a multi-modal data.
  • Violin plots display the whole distribution. They do not display outliers separately as in case of Box plots.

Drawing a violin plot using Python and Matplotlib:

  • To create a violin plot, import the matplotlib.pyplot module and call the method violinplot() function by passing the data as sequences.
  • The violin plot can be customized to display mean and median values.
  • Using Matplotlib both vertical and horizontal violin plots can be created through the parameter vert. By default the value of the vert parameter is True, which creates a vertical violin plot. The width of the violin can be controlled by the widths parameter.

Example:

# Example python program to draw a violin plot using matplotlib

import matplotlib.pyplot as plot

import pandas as pd

import numpy as np

 

# Customized method to draw violin plot in a matplotlib subplot

def drawViolinPlot(axis, xlabel, xticks, xticklabels, ylabel, bandwidth, title):

    # Customize x-axis

    axis.set_xlabel(xlabel);

    axis.set_xticks(xticks);

    axis.set_xticklabels(xticklabels);

 

    # Customize y-axis

    axis.set_ylabel(ylabel);

 

    # Draw a violin plot for the distributions - Default bandwidth

    axis.violinplot(sequences, showmeans=True, showmedians=True, bw_method=bandwidth);

    axis.set_title(title);

 

 

# Distributions as python lists

triangularSeries = [0, 1, 3, 6, 10, 15, 21, 28, 36, 45];

pentagonalSeries = [1, 5, 12, 22, 35, 51, 70, 92, 117, 145];

 

# A tuple of distributions

sequences = (triangularSeries, pentagonalSeries);

 

figure, axis = plot.subplots(2, 1);

plot.subplots_adjust(hspace=1);

 

bandwidth = None;

drawViolinPlot(axis[0],

              "Number Series",

              np.arange(len(sequences)+1),

              ('', 'Triangular Series', 'Pentagonal Series'),

              "Specific number in a series", bandwidth,

              "Violin Plot - Default Bandwidth(i.e., method=scott)");

 

bandwidth = 0.3;

drawViolinPlot(axis[1],

              "Number Series",

              np.arange(len(sequences)+1),

              ('', 'Triangular Series', 'Pentagonal Series'),

              "Specific number in a series", bandwidth,

              "Violin Plot - Bandwidth=0.3(i.e., a scalar value)");             

 

# Display violin plot

plot.show();                 

Output:

Violin plot with bandwidth calculated using scott method

Violin plot with a scalar bandwidth


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