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Table of Contents

Usage Guide

This tutorial covers some basic usage patterns and best-practices to help you get started with Matplotlib.

# sphinx_gallery_thumbnail_number = 3
import matplotlib.pyplot as plt
import numpy as np

A simple example

Matplotlib graphs your data on Figures (i.e., windows, Jupyter widgets, etc.), each of which can contain one or more Axes (i.e., an area where points can be specified in terms of x-y coordinates (or theta-r in a polar plot, or x-y-z in a 3D plot, etc.). The most simple way of creating a figure with an axes is using pyplot.subplots. We can then use Axes.plot to draw some data on the axes:

fig, ax = plt.subplots()  # Create a figure containing a single axes.
ax.plot([1, 2, 3, 4], [1, 4, 2, 3])  # Plot some data on the axes.
../../_images/sphx_glr_usage_001.png

Many other plotting libraries or languages do not require you to explicitly create an axes. For example, in MATLAB, one can just do

plot([1, 2, 3, 4], [1, 4, 2, 3])  % MATLAB plot.

and get the desired graph.

In fact, you can do the same in Matplotlib: for each Axes graphing method, there is a corresponding function in the matplotlib.pyplot module that performs that plot on the "current" axes, creating that axes (and its parent figure) if they don't exist yet. So the previous example can be written more shortly as

plt.plot([1, 2, 3, 4], [1, 4, 2, 3])  # Matplotlib plot.
../../_images/sphx_glr_usage_002.png

Parts of a Figure

Now, let's have a deeper look at the components of a Matplotlib figure.

../../_images/anatomy.png

Figure

The whole figure. The figure keeps track of all the child Axes, a smattering of 'special' artists (titles, figure legends, etc), and the canvas. (Don't worry too much about the canvas, it is crucial as it is the object that actually does the drawing to get you your plot, but as the user it is more-or-less invisible to you). A figure can contain any number of Axes, but will typically have at least one.

The easiest way to create a new figure is with pyplot:

fig = plt.figure()  # an empty figure with no Axes
fig, ax = plt.subplots()  # a figure with a single Axes
fig, axs = plt.subplots(2, 2)  # a figure with a 2x2 grid of Axes

It's convenient to create the axes together with the figure, but you can also add axes later on, allowing for more complex axes layouts.

Axes

This is what you think of as 'a plot', it is the region of the image with the data space. A given figure can contain many Axes, but a given Axes object can only be in one Figure. The Axes contains two (or three in the case of 3D) Axis objects (be aware of the difference between Axes and Axis) which take care of the data limits (the data limits can also be controlled via the set_xlim() and set_ylim() Axes methods). Each Axes has a title (set via set_title()), an x-label (set via set_xlabel()), and a y-label set via set_ylabel()).

The Axes class and it's member functions are the primary entry point to working with the OO interface.

Axis

These are the number-line-like objects. They take care of setting the graph limits and generating the ticks (the marks on the axis) and ticklabels (strings labeling the ticks). The location of the ticks is determined by a Locator object and the ticklabel strings are formatted by a Formatter. The combination of the correct Locator and Formatter gives very fine control over the tick locations and labels.

Artist

Basically everything you can see on the figure is an artist (even the Figure, Axes, and Axis objects). This includes Text objects, Line2D objects, collection objects, Patch objects ... (you get the idea). When the figure is rendered, all of the artists are drawn to the canvas. Most Artists are tied to an Axes; such an Artist cannot be shared by multiple Axes, or moved from one to another.

Types of inputs to plotting functions

All of plotting functions expect np.array or np.ma.masked_array as input. Classes that are 'array-like' such as pandas data objects and np.matrix may or may not work as intended. It is best to convert these to np.array objects prior to plotting.

For example, to convert a pandas.DataFrame

a = pandas.DataFrame(np.random.rand(4,5), columns = list('abcde'))
a_asarray = a.values

and to convert a np.matrix

b = np.matrix([[1, 2], [3, 4]])
b_asarray = np.asarray(b)

The object-oriented interface and the pyplot interface

As noted above, there are essentially two ways to use Matplotlib:

  • Explicitly create figures and axes, and call methods on them (the "object-oriented (OO) style").
  • Rely on pyplot to automatically create and manage the figures and axes, and use pyplot functions for plotting.

So one can do (OO-style)

x = np.linspace(0, 2, 100)

# Note that even in the OO-style, we use `.pyplot.figure` to create the figure.
fig, ax = plt.subplots()  # Create a figure and an axes.
ax.plot(x, x, label='linear')  # Plot some data on the axes.
ax.plot(x, x**2, label='quadratic')  # Plot more data on the axes...
ax.plot(x, x**3, label='cubic')  # ... and some more.
ax.set_xlabel('x label')  # Add an x-label to the axes.
ax.set_ylabel('y label')  # Add a y-label to the axes.
ax.set_title("Simple Plot")  # Add a title to the axes.
ax.legend()  # Add a legend.
../../_images/sphx_glr_usage_003.png

or (pyplot-style)

x = np.linspace(0, 2, 100)

plt.plot(x, x, label='linear')  # Plot some data on the (implicit) axes.
plt.plot(x, x**2, label='quadratic')  # etc.
plt.plot(x, x**3, label='cubic')
plt.xlabel('x label')
plt.ylabel('y label')
plt.title("Simple Plot")
plt.legend()
../../_images/sphx_glr_usage_004.png

Actually there is a third approach, for the case where you are embedding Matplotlib in a GUI application, which completely drops pyplot, even for figure creation. We won't discuss it here; see the corresponding section in the gallery for more info (Embedding Matplotlib in graphical user interfaces).

Matplotlib's documentation and examples use both the OO and the pyplot approaches (which are equally powerful), and you should feel free to use either (however, it is preferrable pick one of them and stick to it, instead of mixing them). In general, we suggest to restrict pyplot to interactive plotting (e.g., in a Jupyter notebook), and to prefer the OO-style for non-interactive plotting (in functions and scripts that are intended to be reused as part of a larger project).

Note

In older examples, you may find examples that instead used the so-called pylab interface, via from pylab import *. This star-import imports everything both from pyplot and from numpy, so that one could do

x = linspace(0, 2, 100)
plot(x, x, label='linear')
...

for an even more MATLAB-like style. This approach is strongly discouraged nowadays and deprecated; it is only mentioned here because you may still encounter it in the wild.

Typically one finds oneself making the same plots over and over again, but with different data sets, which leads to needing to write specialized functions to do the plotting. The recommended function signature is something like:

def my_plotter(ax, data1, data2, param_dict):
    """
    A helper function to make a graph

    Parameters
    ----------
    ax : Axes
        The axes to draw to

    data1 : array
       The x data

    data2 : array
       The y data

    param_dict : dict
       Dictionary of kwargs to pass to ax.plot

    Returns
    -------
    out : list
        list of artists added
    """
    out = ax.plot(data1, data2, **param_dict)
    return out

which you would then use as:

data1, data2, data3, data4 = np.random.randn(4, 100)
fig, ax = plt.subplots(1, 1)
my_plotter(ax, data1, data2, {'marker': 'x'})
../../_images/sphx_glr_usage_005.png

or if you wanted to have 2 sub-plots:

fig, (ax1, ax2) = plt.subplots(1, 2)
my_plotter(ax1, data1, data2, {'marker': 'x'})
my_plotter(ax2, data3, data4, {'marker': 'o'})
../../_images/sphx_glr_usage_006.png

For these simple examples this style seems like overkill, however once the graphs get slightly more complex it pays off.

Backends

What is a backend?

A lot of documentation on the website and in the mailing lists refers to the "backend" and many new users are confused by this term. matplotlib targets many different use cases and output formats. Some people use matplotlib interactively from the python shell and have plotting windows pop up when they type commands. Some people run Jupyter notebooks and draw inline plots for quick data analysis. Others embed matplotlib into graphical user interfaces like wxpython or pygtk to build rich applications. Some people use matplotlib in batch scripts to generate postscript images from numerical simulations, and still others run web application servers to dynamically serve up graphs.

To support all of these use cases, matplotlib can target different outputs, and each of these capabilities is called a backend; the "frontend" is the user facing code, i.e., the plotting code, whereas the "backend" does all the hard work behind-the-scenes to make the figure. There are two types of backends: user interface backends (for use in pygtk, wxpython, tkinter, qt4, or macosx; also referred to as "interactive backends") and hardcopy backends to make image files (PNG, SVG, PDF, PS; also referred to as "non-interactive backends").

There are three ways to configure your backend. If they conflict each other, the method mentioned last in the following list will be used, e.g. calling use() will override the setting in your matplotlibrc.

  1. The rcParams["backend"] parameter in your matplotlibrc file (see Customizing Matplotlib with style sheets and rcParams):

    backend : qt5agg   # use pyqt5 with antigrain (agg) rendering
    

    If no backend is explicitly set in the matplotlibrc file, Matplotlib automatically detects a usable backend based on what is available on your system and on whether a GUI event loop is already running.

    On Linux, if the environment variable DISPLAY is unset, the "event loop" is identified as "headless", which causes a fallback to a noninteractive backend (agg).

  2. Setting the MPLBACKEND environment variable, either for your current shell or for a single script. On Unix:

    > export MPLBACKEND=module://my_backend
    > python simple_plot.py
    
    > MPLBACKEND="module://my_backend" python simple_plot.py
    

    On Windows, only the former is possible:

    > set MPLBACKEND=module://my_backend
    > python simple_plot.py
    

    Setting this environment variable will override the backend parameter in any matplotlibrc, even if there is a matplotlibrc in your current working directory. Therefore setting MPLBACKEND globally, e.g. in your .bashrc or .profile, is discouraged as it might lead to counter-intuitive behavior.

  3. If your script depends on a specific backend you can use the use() function:

    import matplotlib
    matplotlib.use('PS')   # generate postscript output by default
    

    If you use the use function, this should be done before importing matplotlib.pyplot. Calling use after pyplot has been imported may fail to switch the backend and raise an ImportError.

    Using use will require changes in your code if users want to use a different backend. Therefore, you should avoid explicitly calling use unless absolutely necessary.

Note

Backend name specifications are not case-sensitive; e.g., 'GTK3Agg' and 'gtk3agg' are equivalent.

With a typical installation of matplotlib, such as from a binary installer or a linux distribution package, a good default backend will already be set, allowing both interactive work and plotting from scripts, with output to the screen and/or to a file, so at least initially you will not need to use any of the methods given above.

If, however, you want to write graphical user interfaces, or a web application server (Matplotlib in a web application server), or need a better understanding of what is going on, read on. To make things a little more customizable for graphical user interfaces, matplotlib separates the concept of the renderer (the thing that actually does the drawing) from the canvas (the place where the drawing goes). The canonical renderer for user interfaces is Agg which uses the Anti-Grain Geometry C++ library to make a raster (pixel) image of the figure. All of the user interfaces except macosx can be used with agg rendering, e.g., WXAgg, GTK3Agg, QT4Agg, QT5Agg, TkAgg. In addition, some of the user interfaces support other rendering engines. For example, with GTK+ 3, you can also select Cairo rendering (backend GTK3Cairo).

For the rendering engines, one can also distinguish between vector or raster renderers. Vector graphics languages issue drawing commands like "draw a line from this point to this point" and hence are scale free, and raster backends generate a pixel representation of the line whose accuracy depends on a DPI setting.

Here is a summary of the matplotlib renderers (there is an eponymous backend for each; these are non-interactive backends, capable of writing to a file):

Renderer Filetypes Description
AGG png raster graphics -- high quality images using the Anti-Grain Geometry engine
PS ps, eps vector graphics -- Postscript output
PDF pdf vector graphics -- Portable Document Format
SVG svg vector graphics -- Scalable Vector Graphics
Cairo png, ps, pdf, svg raster or vector graphics -- using the Cairo library

To save plots using the non-interactive backends, use the matplotlib.pyplot.savefig('filename') method.

And here are the user interfaces and renderer combinations supported; these are interactive backends, capable of displaying to the screen and of using appropriate renderers from the table above to write to a file:

Backend Description
Qt5Agg Agg rendering in a Qt5 canvas (requires PyQt5). This backend can be activated in IPython with %matplotlib qt5.
ipympl Agg rendering embedded in a Jupyter widget. (requires ipympl). This backend can be enabled in a Jupyter notebook with %matplotlib ipympl.
GTK3Agg Agg rendering to a GTK 3.x canvas (requires PyGObject, and pycairo or cairocffi). This backend can be activated in IPython with %matplotlib gtk3.
macosx Agg rendering into a Cocoa canvas in OSX. This backend can be activated in IPython with %matplotlib osx.
TkAgg Agg rendering to a Tk canvas (requires TkInter). This backend can be activated in IPython with %matplotlib tk.
nbAgg Embed an interactive figure in a Jupyter classic notebook. This backend can be enabled in Jupyter notebooks via %matplotlib notebook.
WebAgg On show() will start a tornado server with an interactive figure.
GTK3Cairo Cairo rendering to a GTK 3.x canvas (requires PyGObject, and pycairo or cairocffi).
Qt4Agg Agg rendering to a Qt4 canvas (requires PyQt4 or pyside). This backend can be activated in IPython with %matplotlib qt4.
WXAgg Agg rendering to a wxWidgets canvas (requires wxPython 4). This backend can be activated in IPython with %matplotlib wx.

ipympl

The Jupyter widget ecosystem is moving too fast to support directly in Matplotlib. To install ipympl

pip install ipympl
jupyter nbextension enable --py --sys-prefix ipympl

or

conda install ipympl -c conda-forge

See jupyter-matplotlib for more details.

GTK and Cairo

GTK3 backends (both GTK3Agg and GTK3Cairo) depend on Cairo (pycairo>=1.11.0 or cairocffi).

How do I select PyQt4 or PySide?

The QT_API environment variable can be set to either pyqt or pyside to use PyQt4 or PySide, respectively.

Since the default value for the bindings to be used is PyQt4, matplotlib first tries to import it, if the import fails, it tries to import PySide.

What is interactive mode?

Use of an interactive backend (see What is a backend?) permits--but does not by itself require or ensure--plotting to the screen. Whether and when plotting to the screen occurs, and whether a script or shell session continues after a plot is drawn on the screen, depends on the functions and methods that are called, and on a state variable that determines whether matplotlib is in "interactive mode". The default Boolean value is set by the matplotlibrc file, and may be customized like any other configuration parameter (see Customizing Matplotlib with style sheets and rcParams). It may also be set via matplotlib.interactive(), and its value may be queried via matplotlib.is_interactive(). Turning interactive mode on and off in the middle of a stream of plotting commands, whether in a script or in a shell, is rarely needed and potentially confusing, so in the following we will assume all plotting is done with interactive mode either on or off.

Note

Major changes related to interactivity, and in particular the role and behavior of show(), were made in the transition to matplotlib version 1.0, and bugs were fixed in 1.0.1. Here we describe the version 1.0.1 behavior for the primary interactive backends, with the partial exception of macosx.

Interactive mode may also be turned on via matplotlib.pyplot.ion(), and turned off via matplotlib.pyplot.ioff().

Note

Interactive mode works with suitable backends in ipython and in the ordinary python shell, but it does not work in the IDLE IDE. If the default backend does not support interactivity, an interactive backend can be explicitly activated using any of the methods discussed in What is a backend?.

Interactive example

From an ordinary python prompt, or after invoking ipython with no options, try this:

import matplotlib.pyplot as plt
plt.ion()
plt.plot([1.6, 2.7])

Assuming you are running version 1.0.1 or higher, and you have an interactive backend installed and selected by default, you should see a plot, and your terminal prompt should also be active; you can type additional commands such as:

plt.title("interactive test")
plt.xlabel("index")

and you will see the plot being updated after each line. Since version 1.5, modifying the plot by other means should also automatically update the display on most backends. Get a reference to the Axes instance, and call a method of that instance:

ax = plt.gca()
ax.plot([3.1, 2.2])

If you are using certain backends (like macosx), or an older version of matplotlib, you may not see the new line added to the plot immediately. In this case, you need to explicitly call draw() in order to update the plot:

plt.draw()

Non-interactive example

Start a fresh session as in the previous example, but now turn interactive mode off:

import matplotlib.pyplot as plt
plt.ioff()
plt.plot([1.6, 2.7])

Nothing happened--or at least nothing has shown up on the screen (unless you are using macosx backend, which is anomalous). To make the plot appear, you need to do this:

plt.show()

Now you see the plot, but your terminal command line is unresponsive; the show() command blocks the input of additional commands until you manually kill the plot window.

What good is this--being forced to use a blocking function? Suppose you need a script that plots the contents of a file to the screen. You want to look at that plot, and then end the script. Without some blocking command such as show(), the script would flash up the plot and then end immediately, leaving nothing on the screen.

In addition, non-interactive mode delays all drawing until show() is called; this is more efficient than redrawing the plot each time a line in the script adds a new feature.

Prior to version 1.0, show() generally could not be called more than once in a single script (although sometimes one could get away with it); for version 1.0.1 and above, this restriction is lifted, so one can write a script like this:

import numpy as np
import matplotlib.pyplot as plt

plt.ioff()
for i in range(3):
    plt.plot(np.random.rand(10))
    plt.show()

which makes three plots, one at a time. I.e. the second plot will show up, once the first plot is closed.

Summary

In interactive mode, pyplot functions automatically draw to the screen.

When plotting interactively, if using object method calls in addition to pyplot functions, then call draw() whenever you want to refresh the plot.

Use non-interactive mode in scripts in which you want to generate one or more figures and display them before ending or generating a new set of figures. In that case, use show() to display the figure(s) and to block execution until you have manually destroyed them.

Performance

Whether exploring data in interactive mode or programmatically saving lots of plots, rendering performance can be a painful bottleneck in your pipeline. Matplotlib provides a couple ways to greatly reduce rendering time at the cost of a slight change (to a settable tolerance) in your plot's appearance. The methods available to reduce rendering time depend on the type of plot that is being created.

Line segment simplification

For plots that have line segments (e.g. typical line plots, outlines of polygons, etc.), rendering performance can be controlled by the path.simplify and path.simplify_threshold parameters in your matplotlibrc file (see Customizing Matplotlib with style sheets and rcParams for more information about the matplotlibrc file). The path.simplify parameter is a boolean indicating whether or not line segments are simplified at all. The path.simplify_threshold parameter controls how much line segments are simplified; higher thresholds result in quicker rendering.

The following script will first display the data without any simplification, and then display the same data with simplification. Try interacting with both of them:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib as mpl

# Setup, and create the data to plot
y = np.random.rand(100000)
y[50000:] *= 2
y[np.logspace(1, np.log10(50000), 400).astype(int)] = -1
mpl.rcParams['path.simplify'] = True

mpl.rcParams['path.simplify_threshold'] = 0.0
plt.plot(y)
plt.show()

mpl.rcParams['path.simplify_threshold'] = 1.0
plt.plot(y)
plt.show()

Matplotlib currently defaults to a conservative simplification threshold of 1/9. If you want to change your default settings to use a different value, you can change your matplotlibrc file. Alternatively, you could create a new style for interactive plotting (with maximal simplification) and another style for publication quality plotting (with minimal simplification) and activate them as necessary. See Customizing Matplotlib with style sheets and rcParams for instructions on how to perform these actions.

The simplification works by iteratively merging line segments into a single vector until the next line segment's perpendicular distance to the vector (measured in display-coordinate space) is greater than the path.simplify_threshold parameter.

Note

Changes related to how line segments are simplified were made in version 2.1. Rendering time will still be improved by these parameters prior to 2.1, but rendering time for some kinds of data will be vastly improved in versions 2.1 and greater.

Marker simplification

Markers can also be simplified, albeit less robustly than line segments. Marker simplification is only available to Line2D objects (through the markevery property). Wherever Line2D construction parameter are passed through, such as matplotlib.pyplot.plot() and matplotlib.axes.Axes.plot(), the markevery parameter can be used:

plt.plot(x, y, markevery=10)

The markevery argument allows for naive subsampling, or an attempt at evenly spaced (along the x axis) sampling. See the Markevery Demo for more information.

Splitting lines into smaller chunks

If you are using the Agg backend (see What is a backend?), then you can make use of the agg.path.chunksize rc parameter. This allows you to specify a chunk size, and any lines with greater than that many vertices will be split into multiple lines, each of which have no more than agg.path.chunksize many vertices. (Unless agg.path.chunksize is zero, in which case there is no chunking.) For some kind of data, chunking the line up into reasonable sizes can greatly decrease rendering time.

The following script will first display the data without any chunk size restriction, and then display the same data with a chunk size of 10,000. The difference can best be seen when the figures are large, try maximizing the GUI and then interacting with them:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib as mpl
mpl.rcParams['path.simplify_threshold'] = 1.0

# Setup, and create the data to plot
y = np.random.rand(100000)
y[50000:] *= 2
y[np.logspace(1,np.log10(50000), 400).astype(int)] = -1
mpl.rcParams['path.simplify'] = True

mpl.rcParams['agg.path.chunksize'] = 0
plt.plot(y)
plt.show()

mpl.rcParams['agg.path.chunksize'] = 10000
plt.plot(y)
plt.show()

Legends

The default legend behavior for axes attempts to find the location that covers the fewest data points (loc='best'). This can be a very expensive computation if there are lots of data points. In this case, you may want to provide a specific location.

Using the fast style

The fast style can be used to automatically set simplification and chunking parameters to reasonable settings to speed up plotting large amounts of data. It can be used simply by running:

import matplotlib.style as mplstyle
mplstyle.use('fast')

It is very light weight, so it plays nicely with other styles, just make sure the fast style is applied last so that other styles do not overwrite the settings:

mplstyle.use(['dark_background', 'ggplot', 'fast'])

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