(For more resources related to this topic, see here.)

Installing matplotlib

Before experimenting with matplotlib, you need to install it. Here we introduce some tips to get matplotlib up and running without too much trouble.

How to do it…

We have three likely scenarios: you might be using Linux, OS X, or Windows.

Linux

Most Linux distributions have Python installed by default, and provide matplotlib in their standard package list. So all you have to do is use the package manager of your distribution to install matplotlib automatically. In addition to matplotlib, we highly recommend that you install NumPy, SciPy, and SymPy, as they are supposed to work together. The following list consists of commands to enable the default packages available in different versions of Linux:

• Ubuntu: The default Python packages are compiled for Python 2.7. In a command terminal, enter the following command:

  sudo apt-get install python-matplotlib python-numpy python-scipy python-sympy
  
  

• ArchLinux: The default Python packages are compiled for Python 3. In a command terminal, enter the following command:

  sudo pacman -S python-matplotlib python-numpy python-scipy python-sympy

  If you prefer using Python 2.7, replace python by python2 in the package names

• Fedora: The default Python packages are compiled for Python 2.7. In a command terminal, enter the following command:

  sudo yum install python-matplotlib numpy scipy sympy

There are other ways to install these packages; in this article, we propose the most simple and seamless ways to do it.

Windows and OS X

Windows and OS X do not have a standard package system for software installation. We have two options—using a ready-made self-installing package or compiling matplotlib from the code source. The second option involves much more work; it is worth the effort to have the latest, bleeding edge version of matplotlib installed. Therefore, in most cases, using a ready-made package is a more pragmatic choice.

You have several choices for ready-made packages: Anaconda, Enthought Canopy, Algorete Loopy, and more! All these packages provide Python, SciPy, NumPy, matplotlib, and more (a text editor and fancy interactive shells) in one go. Indeed, all these systems install their own package manager and from there you install/uninstall additional packages as you would do on a typical Linux distribution. For the sake of brevity, we will provide instructions only for Enthought Canopy. All the other systems have extensive documentation online, so installing them should not be too much of a problem.

So, let’s install Enthought Canopy by performing the following steps:

1. Download the Enthought Canopy installer from https://www.enthought.com/products/canopy. You can choose the free Express edition. The website can guess your operating system and propose the right installer for you.
2. Run the Enthought Canopy installer. You do not need to be an administrator to install the package if you do not want to share the installed software with other users.
3. When installing, just click on Next to keep the defaults. You can find additional information about the installation process at http://docs.enthought.com/canopy/quick-start.html.

That’s it! You will have Python 2.7, NumPy, SciPy, and matplotlib installed and ready to run.

Plotting one curve

The initial example of Hello World! for a plotting software is often about showing a simple curve. We will keep up with that tradition. It will also give you a rough idea about how matplotlib works.

Getting ready

You need to have Python (either v2.7 or v3) and matplotlib installed. You also need to have a text editor (any text editor will do) and a command terminal to type and run commands.

How to do it…

Let’s get started with one of the most common and basic graph that any plotting software offers—curves. In a text file saved as plot.py, we have the following code:

import matplotlib.pyplot as plt

X = range(100)
Y = [value ** 2 for value in X]

plt.plot(X, Y)
plt.show()

Assuming that you installed Python and matplotlib, you can now use Python to interpret this script. If you are not familiar with Python, this is indeed a Python script we have there! In a command terminal, run the script in the directory where you saved plot.py with the following command:

python plot.py

Doing so will open a window as shown in the following screenshot:

The window shows the curve Y = X ** 2 with X in the [0, 99] range. As you might have noticed, the window has several icons, some of which are as follows:

• : This icon opens a dialog, allowing you to save the graph as a picture file. You can save it as a bitmap picture or a vector picture.
• : This icon allows you to translate and scale the graphics. Click on it and then move the mouse over the graph. Clicking on the left button of the mouse will translate the graph according to the mouse movements. Clicking on the right button of the mouse will modify the scale of the graphics.
• : This icon will restore the graph to its initial state, canceling any translation or scaling you might have applied before.

How it works…

Assuming that you are not very familiar with Python yet, let’s analyze the script demonstrated earlier.

The first line tells Python that we are using the matplotlib.pyplot module. To save on a bit of typing, we make the name plt equivalent to matplotlib.pyplot. This is a very common practice that you will see in matplotlib code.

The second line creates a list named X, with all the integer values from 0 to 99. The range function is used to generate consecutive numbers. You can run the interactive Python interpreter and type the command range(100) if you use Python 2, or the command list(range(100)) if you use Python 3. This will display the list of all the integer values from 0 to 99. In both versions, sum(range(100)) will compute the sum of the integers from 0 to 99.

The third line creates a list named Y, with all the values from the list X squared. Building a new list by applying a function to each member of another list is a Python idiom, named list comprehension. The list Y will contain the squared values of the list X in the same order. So Y will contain 0, 1, 4, 9, 16, 25, and so on.

The fourth line plots a curve, where the x coordinates of the curve’s points are given in the list X, and the y coordinates of the curve’s points are given in the list Y. Note that the names of the lists can be anything you like.

The last line shows a result, which you will see on the window while running the script.

There’s more…

So what we have learned so far? Unlike plotting packages like gnuplot, matplotlib is not a command interpreter specialized for the purpose of plotting. Unlike Matlab, matplotlib is not an integrated environment for plotting either. matplotlib is a Python module for plotting. Figures are described with Python scripts, relying on a (fairly large) set of functions provided by matplotlib.

Thus, the philosophy behind matplotlib is to take advantage of an existing language, Python. The rationale is that Python is a complete, well-designed, general purpose programming language. Combining matplotlib with other packages does not involve tricks and hacks, just Python code. This is because there are numerous packages for Python for pretty much any task. For instance, to plot data stored in a database, you would use a database package to read the data and feed it to matplotlib. To generate a large batch of statistical graphics, you would use a scientific computing package such as SciPy and Python’s I/O modules.

Thus, unlike many plotting packages, matplotlib is very orthogonal—it does plotting and only plotting. If you want to read inputs from a file or do some simple intermediary calculations, you will have to use Python modules and some glue code to make it happen. Fortunately, Python is a very popular language, easy to master and with a large user base. Little by little, we will demonstrate the power of this approach.