Pyplot is an API (Application Programming Interface) for Python’s matplotlib that effectively makes matplotlib a viable open source alternative to MATLAB. Matplotlib is a library for data visualization, typically in the form of plots, graphs and charts.

Pyplot API Structure

Pyplot provides matplotlib with two key features:

• A MATLAB-style interface, which allows those familiar with MATLAB to adopt Python more easily
• Statefulness, which means that pyplot stores the state of an object when you first plot it. This is essential for use in the same loop or session state until plt.close() is encountered in the code. State can also be important when creating several plots continuously.  

The pyplot API consists of a hierarchy of Python code objects, and includes numerous functions topped by matplotlib.pyplot. This stack can be viewed as having three interdependent layers:

• Scripting layer – used to define a figure, which contains one or more plots, which consist of axes (i.e., x axis ,y axis, and possibly z axis)
• Artist Layer – used to manipulate elements of a plot, such as adding labels, drawing lines, etc
• Backend Layer – used to format the plot for display in a specific target application, such as a Jupyter Notebook

Scripting Layer

The top-level object in the scripting layer is matplotlib.pyplot.figure(), otherwise known as the Figure. The figure acts as a container that manages all of the elements in a given plot.

Let’s examine each line in the following script to see how pyplot creates a plot and displays it:

import matplotlib.pyplot as plt
plt.plot([1, 2, 3]) 
plt.title(”Simple Plot”) 
plt.show()

1. Imports pyplot() as plt. A configuration file is parsed and a default Figure and Axe are defined.
2. The pyplot plot() command is directed to the matplotlib.axes.Axes.plot function in the backend layer. It provides a unified interface for different types of plots. In this instance, a custom Figure and Axes have not been defined, and plotting will take place in the default Figure and Axes. Pyplot assumes that [1, 2, 3] is a sequence of y values, and automatically generates a corresponding sequence of x values. The x,y values are plotted as coordinates identified as xaxis()and yaxis() function objects contained in the Axes, which is contained by the Figure. Note that a Figure can contain more than one Axes.
3. The pyplot title() command is directed to the Axes.set_title function in the backend layer. This will ensure that the plot is titled “Simple Plot” in this case.
4. The pyplot show() command is directed to the matplotlib.figure.Figure.show() function in the backend layer. It causes the Figure to render based on previous commands in the script, start the UI mainloop, and display the Figure and its elements on the screen.

Artist Layer

The artist layer acts as a descriptive medium between pyplot scripts and more complex Python class objects in the other layers. In this layer, abstract interface classes mediate with high level pyplot elements in the scriptinging layer, represented here as figurative objects in a drawing. 

Artist is analogous to a canvas where scripts in the scripting layer correspond to classes in the backend layer, and is understood as a hierarchy of objects on an abstract canvas. It is here that a sequence of values in a pyplot script can be sketched as a line (Line2D), for example.

Figure 1. A sketch of artist layer elements that represent pyplot elements in the example, ‘Simple Plot’:

Figure 2.  Hierarchy of Artist elements in the example, ‘Simple Plot’:

Backend Layer

The backend layer provides a concrete implementation of the artist layer and its abstraction. The term ‘backend’ is most commonly used to refer to output formats that matplotlib supports. 

Currently supported backends include:

• matplotlib.backends.backend_mixed
  • Backend that implements a renderer that switches between vector and raster drawing. 

• matplotlib.backends.backend_template
  • A do-nothing backend intended as a template for backend developers. 

• matplotlib.backends.backend_agg
  • Backend writes to file, not used for rendering.

• matplotlib.backends.backend_cairo
  • Backend for raster and vector graphics. Uses the Cairo graphics library.

• matplotlib.backends.backend_gtk3agg 
  • Backend for Agg rendering to a GTK 3.x canvas 

• matplotlib.backends.backend_gtk3cairo 
  • Backend for Cairo rendering to a GTK 3.x canvas.

• matplotlib.backends.backend_nbagg 
  • Backend that provides interactive figures for the IPython notebook.

• matplotlib.backends.backend_pdf
  • Backend that provides vector graphics in PDF format.

• matplotlib.backends.backend_pgf
  • Backend that converts a pdf file to a png file.

• matplotlib.backends.backend_ps
  • Backend that produces both PostScript (.ps) and Encapsulated PostScript (.eps) output.

• matplotlib.backends.backend_qt4agg
  • Backend for Agg rendering to a Qt4 canvas 

• matplotlib.backends.backend_qt4cairo
  • Backend for Cairo rendering to a Qt4 canvas.

• matplotlib.backends.backend_qt5ag
  • Backend for Agg rendering in a Qt5 canvas. 

• matplotlib.backends.backend_qt5cairo
  • Backend for Cairo rendering in a Qt5 canvas.

• matplotlib.backends.backend_svg
  • Backend that outputs plot as a Scalable Vector Graphic (SVG).

• matplotlib.backends.backend_tkagg
  • Backend for Agg rendering to a Tk canvas (requires TkInter).

• matplotlib.backends.backend_webagg
  • Backend that starts a tornado server with an interactive Figure, when the show() command in a script is reached.

• Matplotlib.backends.backend_wxagg
  • A wxPython backend for Agg.

The following tutorials will provide you with step-by-step instructions on how to work with Matplotlib, including:

• How to clear a plot in Python
• How to display a plot in Python
• How to build a Numpy array
• How to turn a Numpy array into a list
• What is Matplotlib in Python and how to use it for plotting?

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