[Python library] Matplotlib 101

Fall 2023
Junehyung Lee

Installing the python library

To install the python library, matplotlib, type below line to your terminal.


python -m pip install matplotlib

Loading the library and initiating an empty figure in matplotlib

To load the installed python libary: (include this line before use plt in the script)

import matplotlib.pyplot as plt

To initiate an empty figure with single ax and a size of 8" x 6":


fig, ax = plt.subplots(figsize=(8, 6))

To initiate an empty figure with multiple axes
(e.g., 6 axes with 2 x 3 layout):

fig, axes = plt.subplots(nrows=2, ncols=3, figsize=(8, 6))

Types of plots that are useful in matplotlib

  • plot: useful when looking at a single feature over a time (i.e., time series data).
  • scatter: useful when looking at two features and relationship between them.
  • axline, axhline, axvline

    • Examples

    "mtcars" dataset has been used for the following examples. The dataset also can be obtained at this url.

    import pandas as pd

    data = pd.read_csv('mtcars.csv')

    Matplotlib scatter

    import matplotlib.pyplot as plt

    fig, ax = plt.subplots(figsize=(8, 6))

    ax.scatter(x=data['Weight'], y=data['MPG'])

    plt.tight_layout()
    plt.show()

    plt.savefig('mtcars-wt_mpg.png', dpi=300)

    plt.close() # Run this line to close the plot.

    Adding labels for x-axis and y-axis

    import matplotlib.pyplot as plt

    fig, ax = plt.subplots(figsize=(8, 3))

    ax.scatter(x=data['Weight'], y=data['MPG'])

    ax.set_xlabel('Weight of vehicle (per 1,000 lbs)')
    ax.set_ylabel('Milers per gallon (mpg)')

    ax.tick_params(axis='both', labelsize=15)

    plt.tight_layout()
    plt.show()

    plt.savefig('mtcars-wt_mpg-with labels.png', dpi=300)

    Plotting them in different colors

    Let's say we want to plot them in differnet colors based on whether its engine is V-shape or not.

    import matplotlib.pyplot as plt

    fig, ax = plt.subplots(figsize=(8, 3))

    ax.scatter(x=data['Weight'], y=data['MPG'], label=data['V-shape'])

    ax.set_xlabel('Weight of vehicle (per 1,000 lbs)')
    ax.set_ylabel('Milers per gallon (mpg)')

    ax.tick_params(axis='both', labelsize=15)

    plt.tight_layout()
    plt.show()

    plt.savefig('mtcars-wt_mpg-with labels.png', dpi=300)