Matplotlib is a data visualization library in Python. Within the library, the pyplot module is commonly used for creating graphs and figures. Here are some reasons that make Matplotlib great:

it allows you to develop professional plots using minimal lines of code
highly customizable and lots of control over line styles, colors and font etc.
performant even when working with large arrays as it can be used together with NumPy

Importing Matplotlib

To import the matplotlib.pyplot module:


        
        
            
                
                
                    import matplotlib.pyplot as plt

Note that, by convention, we always use the alias plt for the pyplot module.

Terminology

First it is important to understand the components of a figure in Matplotlib. The components are hierarchical:

Figure object: the whole figure
Axes object: belongs to a Figure object and is the space where we add the data to visualize
Axis object: belongs to an Axes object. Axis object can be categorized as XAxis or YAxis.

The following figure illustrates the hierarchical relationship between the three objects:

The following figure also shows some of the more detailed elements composing a figure in Matplotlib:

Approaches to plotting

There are two main approaches to plotting in Matplotlib:

Object-Oriented Interface
Pyplot Interface

NOTE

If you need granular control over how a graph is to be plotted, then the Object-Oriented approach is recommended as it provides more flexibility and control.

Object-Oriented Interface

In this approach, we create Figure objects that will hold Axes objects (i.e. plots). We do this explicitly, and this in turn allows for a greater level of control and customization when constructing our figures.

The following code snippet demonstrates how to plot a line graph using the object-oriented interface:


        
        
            
                
                
                    import matplotlib.pyplot as plt

# Initialize a Figure object
fig = plt.figure()

# Add an Axes object to the Figure using add_subplot(1,1,1)
# (1,1,1) here tells to add a 1x1 grid, 1st subplot
ax = fig.add_subplot(1,1,1)

x = [1,2,3]
y = [1,2,3]

# Plot the data and show it
ax.plot(x, y)
plt.show()

This renders the following graph:

Instead of generating the Figure and Axes object separately, we can initialize them together:


        
        
            
                
                
                    fig, ax = plt.subplots()
ax.plot(x, y)
plt.show()

Pyplot Interface

The Pyplot interface will automatically generate and set the Figure / Axes objects you work with for you. This means you will continue to layer on changes to the same Figure using pyplot commands. This is different to the Object-Oriented interface where we explicitly specify which Figure or Axes object to apply our changes to.

WARNING

Although the Pyplot interface is a convenient and fast way to plot simple graphs, it isn't suited for making more complicated plots such as subplots.

Using the Pyplot interface, we are able to easily create simple figures. For example, to create a basic line chart:


        
        
            
                
                
                    x = [1,2,3]
y = [1,2,3]
plt.plot(x,y)
plt.show()

This generates the following plot:

In this case, the plt.plot(~) call is implicitly creating a Figure instance and an Axes instance in the background.

Basics

Drawing a bar chart

To draw a bar chart using the Pyplot interface:


        
        
            
                
                
                    plt.bar(["Blue", "Red"], [2,5])
plt.show()

To perform the same using the object-oriented approach:


        
        
            
                
                
                    fig, ax = plt.subplots()
ax.bar(["Blue", "Red"], [2,5])
plt.show()

Both pieces of code result in a bar chart like below:

Drawing a histogram

To draw a histogram showing the distribution of some numbers using the Pyplot interface:


        
        
            
                
                
                    nums = [1,1,2,3,3,3,3,3,4,5,6,6,6,7,8,8,9,10,12,12,12,12,14,18]
_ = plt.hist(nums)
_ = plt.xlabel('Number')
_ = plt.ylabel('Frequency')
plt.show()

To perform the same using the object-oriented approach:


        
        
            
                
                
                    nums = [1,1,2,3,3,3,3,3,4,5,6,6,6,7,8,8,9,10,12,12,12,12,14,18]
fig,ax = plt.subplots()
ax.hist(nums)
ax.set_xlabel('Number')
ax.set_ylabel('Frequency')
plt.show()

Both pieces of code result in a histogram as follows:

Drawing a scatter plot

To draw a basic 2D scatter plot using the Pyplot interface:


        
        
            
                
                
                    plt.scatter([5,2,3], [1,2,4])
plt.show()

To draw the same 2D scatter plot using the object-oriented interface:


        
        
            
                
                
                    fig, ax = plt.subplots()
ax.scatter([5,2,3], [1,2,4])
plt.show()

Both pieces of code result in a scatter plot like the below:

Adding a title

To add a title to a plot using Pyplot interface:


        
        
            
                
                
                    plt.plot([1,2])
plt.title("My Graph")
plt.show()

To perform the same using the object-oriented interface:


        
        
            
                
                
                    fig, ax = plt.subplots()
ax.plot([1,2])
ax.set_title("My Graph")
plt.show()

Both pieces of code produce the following plot:

Adding axis labels

To add axis labels using the Pyplot interface:


        
        
            
                
                
                    plt.plot([1,2])
plt.xlabel("X Axis")
plt.ylabel("Y Axis")
plt.show()

To perform the same using the object-oriented interface:


        
        
            
                
                
                    fig, ax = plt.subplots()
ax.plot([1,2])
ax.set_xlabel("X Axis")
ax.set_ylabel("Y Axis")
plt.show()

Both pieces of code produce the following plot with labeled X and Y Axis:

Adding a legend

To add a legend using the Pyplot interface:


        
        
            
                
                
                    plt.plot([1,2,3], label="blue", color="blue")   # add the label parameter here
plt.plot([3,2,1], label="red", color="red")     # add the label parameter here
plt.legend()   # to embed the legend
plt.show()

To perform the same using the object-oriented interface:


        
        
            
                
                
                    fig, ax = plt.subplots()
ax.plot([1,2,3], label="blue", color="blue")
ax.plot([3,2,1], label="red", color="red")
ax.legend()
plt.show()

Both pieces of code produce the following:

Adding grid lines

To show grid lines using the Pyplot interface use the plot.grid(~) method:


        
        
            
                
                
                    plt.plot([1,2,3])
plt.grid(color="gray", linestyle="--", linewidth=0.7)
plt.show()

To perform the same using the object oriented interface:


        
        
            
                
                
                    fig, ax = plt.subplots()
ax.plot([1,2,3])
ax.grid(color="gray", linestyle="--", linewidth=0.7)
plt.show()

Both pieces of code produce the following:

Saving figures

To save a figure in Matplotlib as a png file:


        
        
            
                
                
                    plt.plot([1,2])   # Your graph
plt.savefig("my_curve.png")

This will save a file named "my_curve.png" in the same directory as your Python script.

To save a figure in Matplotlib as a pdf file:


        
        
            
                
                
                    plt.plot([1,2])   # Your graph
plt.savefig("my_curve.pdf")

This will save a file named "my_curve.pdf" in the same directory as your Python script.

Published by Arthur Yanagisawa

Edited by 0 others

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