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# Usage Guide

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

## General Concepts

`matplotlib` has an extensive codebase that can be daunting to many new users. However, most of matplotlib can be understood with a fairly simple conceptual framework and knowledge of a few important points.

Plotting requires action on a range of levels, from the most general (e.g., ‘contour this 2-D array’) to the most specific (e.g., ‘color this screen pixel red’). The purpose of a plotting package is to assist you in visualizing your data as easily as possible, with all the necessary control – that is, by using relatively high-level commands most of the time, and still have the ability to use the low-level commands when needed.

Therefore, everything in matplotlib is organized in a hierarchy. At the top of the hierarchy is the matplotlib “state-machine environment” which is provided by the `matplotlib.pyplot` module. At this level, simple functions are used to add plot elements (lines, images, text, etc.) to the current axes in the current figure.

Note

Pyplot’s state-machine environment behaves similarly to MATLAB and should be most familiar to users with MATLAB experience.

The next level down in the hierarchy is the first level of the object-oriented interface, in which pyplot is used only for a few functions such as figure creation, and the user explicitly creates and keeps track of the figure and axes objects. At this level, the user uses pyplot to create figures, and through those figures, one or more axes objects can be created. These axes objects are then used for most plotting actions.

For even more control – which is essential for things like embedding matplotlib plots in GUI applications – the pyplot level may be dropped completely, leaving a purely object-oriented approach.

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

## Parts of a Figure

### `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 have any number of `Axes`, but to be useful should 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.suptitle('No axes on this figure')  # Add a title so we know which it is

    fig, ax_lst = plt.subplots(2, 2)  # a figure with a 2x2 grid of Axes

### `Axes`

This is what you think of as ‘a plot’, it is the region of the image with the data space (marked as the inner blue box). 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 set 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 (circled in green). 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_asndarray = a.values

and to covert a `np.matrix`

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

## Matplotlib, pyplot and pylab: how are they related?

Matplotlib is the whole package; `matplotlib.pyplot` is a module in matplotlib; and `pylab` is a module that gets installed alongside `matplotlib`.

Pyplot provides the state-machine interface to the underlying object-oriented plotting library. The state-machine implicitly and automatically creates figures and axes to achieve the desired plot. For example:

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

    plt.plot(x, x, label='linear')
    plt.plot(x, x**2, label='quadratic')
    plt.plot(x, x**3, label='cubic')

    plt.xlabel('x label')
    plt.ylabel('y label')

    plt.title("Simple Plot")

    plt.legend()

    plt.show()

The first call to `plt.plot` will automatically create the necessary figure and axes to achieve the desired plot. Subsequent calls to `plt.plot` re-use the current axes and each add another line. Setting the title, legend, and axis labels also automatically use the current axes and set the title, create the legend, and label the axis respectively.

`pylab` is a convenience module that bulk imports `matplotlib.pyplot` (for plotting) and `numpy` (for mathematics and working with arrays) in a single name space. Although many examples use `pylab`, it is no longer recommended.

For non-interactive plotting it is suggested to use pyplot to create the figures and then the OO interface for plotting.

## Coding Styles

When viewing this documentation and examples, you will find different coding styles and usage patterns. These styles are perfectly valid and have their pros and cons. Just about all of the examples can be converted into another style and achieve the same results. The only caveat is to avoid mixing the coding styles for your own code.

Note

Developers for matplotlib have to follow a specific style and guidelines. See The Matplotlib Developers’ Guide.

Of the different styles, there are two that are officially supported. Therefore, these are the preferred ways to use matplotlib.

For the pyplot style, the imports at the top of your scripts will typically be:

    import matplotlib.pyplot as plt
    import numpy as np

Then one calls, for example, np.arange, np.zeros, np.pi, plt.figure, plt.plot, plt.show, etc. Use the pyplot interface for creating figures, and then use the object methods for the rest:

    x = np.arange(0, 10, 0.2)
    y = np.sin(x)
    fig = plt.figure()
    ax = fig.add_subplot(111)
    ax.plot(x, y)
    plt.show()

So, why all the extra typing instead of the MATLAB-style (which relies on global state and a flat namespace)? For very simple things like this example, the only advantage is academic: the wordier styles are more explicit, more clear as to where things come from and what is going on. For more complicated applications, this explicitness and clarity becomes increasingly valuable, and the richer and more complete object-oriented interface will likely make the program easier to write and maintain.

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'})

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'})

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

## 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 embed matplotlib into graphical user interfaces like wxpython or pygtk to build rich applications. Others use matplotlib in batch scripts to generate postscript images from some numerical simulations, and still others in 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 four 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 `backend` parameter in your `matplotlibrc` file (see Customizing matplotlib):

       backend : WXAgg   # use wxpython with antigrain (agg) rendering

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

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

       > 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 must be done before importing `matplotlib.pyplot`. Calling `use()` after pyplot has been imported will have no effect. 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., ‘GTKAgg’ and ‘gtkagg’ 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`, `GTKAgg`, `QT4Agg`, `QT5Agg`, `TkAgg`. In addition, some of the user interfaces support other rendering engines. For example, with GTK, you can also select GDK rendering (backend `GTK` deprecated in 2.0) or Cairo rendering (backend `GTKCairo`).

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 backed 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 … | vector graphics – Cairo graphics
GDK | png jpg tiff … | raster graphics – the Gimp Drawing Kit Deprecated in 2.0

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
--- | ---
GTKAgg | Agg rendering to a GTK 2.x canvas (requires PyGTK and pycairo or cairocffi; Python2 only)
GTK3Agg | Agg rendering to a GTK 3.x canvas (requires PyGObject and pycairo or cairocffi)
GTK | GDK rendering to a GTK 2.x canvas (not recommended and d eprecated in 2.0) (requires PyGTK and pycairo or cairocffi; Python2 only)
GTKCairo | Cairo rendering to a GTK 2.x canvas (requires PyGTK and pycairo or cairocffi; Python2 only)
GTK3Cairo | Cairo rendering to a GTK 3.x canvas (requires PyGObject and pycairo or cairocffi)
WXAgg | Agg rendering to a wxWidgets canvas (requires wxPython)
WX | Native wxWidgets drawing to a wxWidgets Canvas (not recommended and deprecated in 2.0) (requires wxPython)
TkAgg | Agg rendering to a Tk canvas (requires TkInter)
Qt4Agg | Agg rendering to a Qt4 canvas (requires PyQt4 or `pyside`)
Qt5Agg | Agg rendering in a Qt5 canvas (requires PyQt5)
macosx | Cocoa rendering in OSX windows (presently lacks blocking show() behavior when matplotlib is in non-interactive mode)

## WX backends

At present the release version of `wxPython` (also known as wxPython classic) does not support python3. A work in progress redesigned version known as wxPython-Phoenix does support python3. Matplotlib should work with both versions.

## GTK and Cairo

Both `GTK2` and `GTK3` have implicit dependencies on PyCairo regardless of the specific Matplotlib backend used. Unfortunatly the latest release of PyCairo for Python3 does not implement the Python wrappers needed for the `GTK3Agg` backend. `Cairocffi` can be used as a replacement which implements the correct wrapper.

## How do I select PyQt4 or PySide?

You can choose either PyQt4 or PySide when using the `qt4` backend by setting the appropriate value for `backend.qt4` in your `matplotlibrc` file. The default value is `PyQt4`.

The setting in your `matplotlibrc` file can be overridden by setting the `QT_API` environment variable 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). 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. This is because you are in interactive mode and you are using pyplot functions. Now try an alternative method of modifying the plot. Get a reference to the `Axes` instance, and call a method of that instance:

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

Nothing changed, because the Axes methods do not include an automatic call to `draw_if_interactive()`; that call is added by the pyplot functions. If you are using methods, then when you want to update the plot on the screen, you need to call `draw()`:

    plt.draw()

Now you should see the new line added to the plot.

### 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.

### 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.

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