matplotlib - 2D and 3D plotting in Python
Adapted from a notebook by J.R. Johansson (jrjohansson at gmail.com)
The latest version of this IPython notebook lecture is available at http://github.com/jrjohansson/scientific-python-lectures.
The other notebooks in this lecture series are indexed at http://jrjohansson.github.io.
Introduction
Matplotlib is an excellent 2D and 3D graphics library for generating scientific figures. Some of the many advantages of this library include:
Easy to get started
Great control of every element in a figure, including figure size and DPI.
High-quality output in many formats, including PNG, PDF, SVG, EPS, and PGF.
GUI for interactively exploring figures and support for headless generation of figure files (useful for batch jobs).
One of the key features of matplotlib that I would like to emphasize, and that I think makes matplotlib highly suitable for generating figures for scientific publications is that all aspects of the figure can be controlled programmatically. This is important for reproducibility and convenient when one needs to regenerate the figure with updated data or change its appearance.
More information at the Matplotlib web page: http://matplotlib.org/
To get started using Matplotlib in a Python program, either include the symbols from the pylab
module (the easy way):
or import the matplotlib.pyplot
module under the name plt
(the tidy way):
MATLAB-like API
The easiest way to get started with plotting using matplotlib is often to use the MATLAB-like API provided by matplotlib.
It is designed to be compatible with MATLAB's plotting functions, so it is easy to get started with if you are familiar with MATLAB.
To use this API from matplotlib, we need to include the symbols in the pylab
module:
Example
A simple figure with MATLAB-like plotting API:
Most of the plotting related functions in MATLAB are covered by the pylab
module. For example, subplot and color/symbol selection:
The good thing about the pylab MATLAB-style API is that it is easy to get started with if you are familiar with MATLAB, and it has a minumum of coding overhead for simple plots.
However, I'd encourrage not using the MATLAB compatible API for anything but the simplest figures.
Instead, I recommend learning and using matplotlib's object-oriented plotting API. It is remarkably powerful. For advanced figures with subplots, insets and other components it is very nice to work with.
The matplotlib object-oriented API
The main idea with object-oriented programming is to have objects that one can apply functions and actions on, and no object or program states should be global (such as the MATLAB-like API). The real advantage of this approach becomes apparent when more than one figure is created, or when a figure contains more than one subplot.
To use the object-oriented API we start out very much like in the previous example, but instead of creating a new global figure instance we store a reference to the newly created figure instance in the fig
variable, and from it we create a new axis instance axes
using the add_axes
method in the Figure
class instance fig
:
Although a little bit more code is involved, the advantage is that we now have full control of where the plot axes are placed, and we can easily add more than one axis to the figure:
If we don't care about being explicit about where our plot axes are placed in the figure canvas, then we can use one of the many axis layout managers in matplotlib. My favorite is subplots
, which can be used like this:
That was easy, but it isn't so pretty with overlapping figure axes and labels, right?
We can deal with that by using the fig.tight_layout
method, which automatically adjusts the positions of the axes on the figure canvas so that there is no overlapping content:
Figure size, aspect ratio and DPI
Matplotlib allows the aspect ratio, DPI and figure size to be specified when the Figure
object is created, using the figsize
and dpi
keyword arguments. figsize
is a tuple of the width and height of the figure in inches, and dpi
is the dots-per-inch (pixel per inch). To create an 800x400 pixel, 100 dots-per-inch figure, we can do:
The same arguments can also be passed to layout managers, such as the subplots
function:
Saving figures
To save a figure to a file we can use the savefig
method in the Figure
class:
Here we can also optionally specify the DPI and choose between different output formats:
What formats are available and which ones should be used for best quality?
Matplotlib can generate high-quality output in a number formats, including PNG, JPG, EPS, SVG, PGF and PDF. For scientific papers, I recommend using PDF whenever possible. (LaTeX documents compiled with pdflatex
can include PDFs using the includegraphics
command). In some cases, PGF can also be good alternative.
Legends, labels and titles
Now that we have covered the basics of how to create a figure canvas and add axes instances to the canvas, let's look at how decorate a figure with titles, axis labels, and legends.
Figure titles
A title can be added to each axis instance in a figure. To set the title, use the set_title
method in the axes instance:
Axis labels
Similarly, with the methods set_xlabel
and set_ylabel
, we can set the labels of the X and Y axes:
Legends
Legends for curves in a figure can be added in two ways. One method is to use the legend
method of the axis object and pass a list/tuple of legend texts for the previously defined curves:
The method described above follows the MATLAB API. It is somewhat prone to errors and unflexible if curves are added to or removed from the figure (resulting in a wrongly labelled curve).
A better method is to use the label="label text"
keyword argument when plots or other objects are added to the figure, and then using the legend
method without arguments to add the legend to the figure:
The advantage with this method is that if curves are added or removed from the figure, the legend is automatically updated accordingly.
The legend
function takes an optional keyword argument loc
that can be used to specify where in the figure the legend is to be drawn. The allowed values of loc
are numerical codes for the various places the legend can be drawn. See http://matplotlib.org/users/legend_guide.html#legend-location for details. Some of the most common loc
values are:
The following figure shows how to use the figure title, axis labels and legends described above:
Setting colors, linewidths, linetypes
Colors
With matplotlib, we can define the colors of lines and other graphical elements in a number of ways. First of all, we can use the MATLAB-like syntax where 'b'
means blue, 'g'
means green, etc. The MATLAB API for selecting line styles are also supported: where, for example, 'b.-' means a blue line with dots:
We can also define colors by their names or RGB hex codes and optionally provide an alpha value using the color
and alpha
keyword arguments:
Line and marker styles
To change the line width, we can use the linewidth
or lw
keyword argument. The line style can be selected using the linestyle
or ls
keyword arguments:
Control over axis appearance
The appearance of the axes is an important aspect of a figure that we often need to modify to make a publication quality graphics. We need to be able to control where the ticks and labels are placed, modify the font size and possibly the labels used on the axes. In this section we will look at controling those properties in a matplotlib figure.
Plot range
The first thing we might want to configure is the ranges of the axes. We can do this using the set_ylim
and set_xlim
methods in the axis object, or axis('tight')
for automatrically getting "tightly fitted" axes ranges:
Logarithmic scale
It is also possible to set a logarithmic scale for one or both axes. This functionality is in fact only one application of a more general transformation system in Matplotlib. Each of the axes' scales are set seperately using set_xscale
and set_yscale
methods which accept one parameter (with the value "log" in this case):
Axis position adjustments
Unfortunately, when saving figures the labels are sometimes clipped, and it can be necessary to adjust the positions of axes a little bit. This can be done using subplots_adjust
:
Axis grid
With the grid
method in the axis object, we can turn on and off grid lines. We can also customize the appearance of the grid lines using the same keyword arguments as the plot
function:
Twin axes
Sometimes it is useful to have dual x or y axes in a figure; for example, when plotting curves with different units together. Matplotlib supports this with the twinx
and twiny
functions:
Other 2D plot styles
In addition to the regular plot
method, there are a number of other functions for generating different kind of plots. See the matplotlib plot gallery for a complete list of available plot types: http://matplotlib.org/gallery.html. Some of the more useful ones are show below:
Text annotation
Annotating text in matplotlib figures can be done using the text
function. It supports LaTeX formatting just like axis label texts and titles:
Figures with multiple subplots and insets
Axes can be added to a matplotlib Figure canvas manually using fig.add_axes
or using a sub-figure layout manager such as subplots
, subplot2grid
, or gridspec
:
subplots
Colormap and contour figures
Colormaps and contour figures are useful for plotting functions of two variables. In most of these functions we will use a colormap to encode one dimension of the data. There are a number of predefined colormaps. It is relatively straightforward to define custom colormaps. For a list of pre-defined colormaps, see: http://www.scipy.org/Cookbook/Matplotlib/Show_colormaps
pcolor
imshow
contour
3D figures
To use 3D graphics in matplotlib, we first need to create an instance of the Axes3D
class. 3D axes can be added to a matplotlib figure canvas in exactly the same way as 2D axes; or, more conveniently, by passing a projection='3d'
keyword argument to the add_axes
or add_subplot
methods.
Surface plots
Coutour plots with projections
Change the view angle
We can change the perspective of a 3D plot using the view_init
method, which takes two arguments: elevation
and azimuth
angle (in degrees):
Further reading
http://www.matplotlib.org - The project web page for matplotlib.
https://github.com/matplotlib/matplotlib - The source code for matplotlib.
http://matplotlib.org/gallery.html - A large gallery showcaseing various types of plots matplotlib can create. Highly recommended!
http://www.loria.fr/~rougier/teaching/matplotlib - A good matplotlib tutorial.
http://scipy-lectures.github.io/matplotlib/matplotlib.html - Another good matplotlib reference.
Exercise: Plot our Champaign-Urbana daily weather data
Here is our code to load in our daily weather data into a pandas
data frame called daily_data
, and perform quality control.
Now we can plot!