Python - Matplotlib

Table of Contents

Background

Matplotlib is a popular python package for figure plot and data visualization.

Installation

pip install matplotlib

Submodules

Matplotlib comes with a series of submodules, e.g.,

  • matplotlib.pyplot is the counterpart of Matlab in matplotlib, which is essentially a collection of functions similar to Matlab.
  • matplotlib.pylab is a convenience module which imports matplotlib.pyplot (for plotting) and numpy (for array related operations) in a single name space.

General Concepts

In Matplotlib, there are some important concepts which can be illustrated in Figure 1. A typical matplotlib Figure is comprised of a variety of components, as show in 2.

matplotlib_fig_concept.png

Figure 1: Concepts in a figure in Matplotlib (from https://matplotlib.org/1.5.1/faq/usage_faq.html)

matplotlib_fig_component.png

Figure 2: Components of a figure in Matplotlib (from https://matplotlib.org/stable/tutorials/introductory/usage.html)

Figure

  • A Figure object is essentially a canvas on which plot operations are performed. Clearly, it consists of one or more Axes objects. 

    import matplotlib.pyplot as plt

# Create an empty Figure object without any Axes.
fig = plt.figure()
fig = plt.figure(figsize=(WIDTH, HEIGHT), dpi=DPI)

# Create a Figure object including one Axes.
fig, ax = plt.subplots()

# Create a Figure object including 2 x 3 Axes.
fig, ax = plt.subplots(2, 3)
  • matplotlib.pyplot.gcf() can get the current Figure object.

Axes

  • Create Axes objects.
    • matplotlib.pyplot.subplots
    • figure.add_axes
  • A Axes object consists of a multiplicity Axis objects, e.g.,
    • 2 Axis objects in a 2-dimension plot, e.g.., X-axis and Y-axis.
    • 3 Axis objects in a 3-dimension plot, e.g.., X-axis, Y-axis, and Z-axis.
  • A Figure object's member function gca() can get the current Axes object.
  • A Figure object's member function subplots can create one or more Axes objects.
  • Member functions
    • plot(X, Y)
    • set_xlabel("XLABEL")
    • set_ylabel("YLABEL")
    • set_title("TITLE")
    • set_xticks("XTICKS")
    • set_yticks("YTICKS")
    • set_xlim([XMIN, XMAX])
    • set_ylim([YMIN, YMAX])
    • legend()
    • set_prop_cycle() : Set the random seed of colors, e.g., None for reset to the default.

Axis

Just as its name indicates, a Axis object defines the properties and attributes of a axis, e.g., limits, ticks, and ticklabels.

Interfaces

As written in the tutorial, there are a couple of API styles provided by matplotlib.

Object-oriented (OO) style

Figures and Axes objects are created and their member functions are provoked explicitly.

Pyplot style

matplotlib.pyplot directly provokes its own functions, e.g., plot, xlabel, ylabel, title, legend, etc.

Miscellaneous

Animation

In submodule matplotlib.animation, Animation is a base class for animation making. It has a couple of derived classes.

FuncAnimation

This class generates an animation by repeatedly calling a function. 

class matplotlib.animation.FuncAnimation(fig, func, frames=None, init_func=None, fargs=None, save_count=None, *, cache_frame_data=True, **kwargs)

where

  • fig is a matplotlib.figure.Figure object.
  • func is a function to call at each frame. Its first argument will be the next value in frames, and other positional arguments can be supplied via fargs parameter. Its signature can be def func(frame, *fargs) -> iterable_of_artists.
  • frames is an iterable object. Particularly, range(frames) can be simplified by assigning an integer.

Its member function save can save a FuncAnimation object to gif, mp4, etc.

ArtistAnimation

This class produces an animation using a fixed set of Artist object.

Default settings

import matplotlib.pyplot as plt

plt.rcParams['font.family'] = "Consolas"
plt.rcParams['font.size'] = 12
plt.rcParams['savefig.dpi'] = 300
plt.rcParams['savefig.bbox'] = "tight"
plt.rcParams['text.usetex'] = True

References