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# Pyplot tutorial

An introduction to the pyplot interface.

## Intro to pyplot

`matplotlib.pyplot` is a collection of command style functions that make matplotlib work like MATLAB. Each `pyplot` function makes some change to a figure: e.g., creates a figure, creates a plotting area in a figure, plots some lines in a plotting area, decorates the plot with labels, etc.

In `matplotlib.pyplot` various states are preserved across function calls, so that it keeps track of things like the current figure and plotting area, and the plotting functions are directed to the current axes (please note that “axes” here and in most places in the documentation refers to the axes part of a figure and not the strict mathematical term for more than one axis).

Note

the pyplot API is generally less-flexible than the object-oriented API. Most of the function calls you see here can also be called as methods from an `Axes` object. We recommend browsing the tutorials and examples to see how this works.

Generating visualizations with pyplot is very quick:

    import matplotlib.pyplot as plt
    plt.plot([1, 2, 3, 4])
    plt.ylabel('some numbers')
    plt.show()

You may be wondering why the x-axis ranges from 0-3 and the y-axis from 1-4. If you provide a single list or array to the `plot()` command, matplotlib assumes it is a sequence of y values, and automatically generates the x values for you. Since python ranges start with 0, the default x vector has the same length as y but starts with 0. Hence the x data are `[0,1,2,3]`.

`plot()` is a versatile command, and will take an arbitrary number of arguments. For example, to plot x versus y, you can issue the command:

    .. code-block:: python

> plt.plot([1, 2, 3, 4], [1, 4, 9, 16])

### Formatting the style of your plot

For every x, y pair of arguments, there is an optional third argument which is the format string that indicates the color and line type of the plot. The letters and symbols of the format string are from MATLAB, and you concatenate a color string with a line style string. The default format string is ‘b-‘, which is a solid blue line. For example, to plot the above with red circles, you would issue

    plt.plot([1, 2, 3, 4], [1, 4, 9, 16], 'ro')
    plt.axis([0, 6, 0, 20])
    plt.show()

See the `plot()` documentation for a complete list of line styles and format strings. The `axis()` command in the example above takes a list of `[xmin, xmax, ymin, ymax]` and specifies the viewport of the axes.

If matplotlib were limited to working with lists, it would be fairly useless for numeric processing. Generally, you will use numpy arrays. In fact, all sequences are converted to numpy arrays internally. The example below illustrates a plotting several lines with different format styles in one command using arrays.

    import numpy as np

    # evenly sampled time at 200ms intervals
    t = np.arange(0., 5., 0.2)

    # red dashes, blue squares and green triangles
    plt.plot(t, t, 'r--', t, t**2, 'bs', t, t**3, 'g^')
    plt.show()

## Plotting with keyword strings

There are some instances where you have data in a format that lets you access particular variables with strings. For example, with `numpy.recarray` or `pandas.DataFrame`.

Matplotlib allows you provide such an object with the `data` keyword argument. If provided, then you may generate plots with the strings corresponding to these variables.

    data = {'a': np.arange(50),
            'c': np.random.randint(0, 50, 50),
            'd': np.random.randn(50)}
    data['b'] = data['a'] + 10 * np.random.randn(50)
    data['d'] = np.abs(data['d']) * 100

    plt.scatter('a', 'b', c='c', s='d', data=data)
    plt.xlabel('entry a')
    plt.ylabel('entry b')
    plt.show()

## Plotting with categorical variables

It is also possible to create a plot using categorical variables. Matplotlib allows you to pass categorical variables directly to many plotting functions. For example:

    names = ['group_a', 'group_b', 'group_c']
    values = [1, 10, 100]

    plt.figure(1, figsize=(9, 3))

    plt.subplot(131)
    plt.bar(names, values)
    plt.subplot(132)
    plt.scatter(names, values)
    plt.subplot(133)
    plt.plot(names, values)
    plt.suptitle('Categorical Plotting')
    plt.show()

## Controlling line properties

Lines have many attributes that you can set: linewidth, dash style, antialiased, etc; see `matplotlib.lines.Line2D`. There are several ways to set line properties

- Use keyword args:

      plt.plot(x, y, linewidth=2.0)

- Use the setter methods of a `Line2D` instance. `plot` returns a list of `Line2D` objects; e.g., `line1, line2 = plot(x1, y1, x2, y2)`. In the code below we will suppose that we have only one line so that the list returned is of length 1. We use tuple unpacking with `line,` to get the first element of that list:

      line, = plt.plot(x, y, '-')
      line.set_antialiased(False) # turn off antialising

- Use the `setp()` command. The example below uses a MATLAB-style command to set multiple properties on a list of lines. `setp` works transparently with a list of objects or a single object. You can either use python keyword arguments or MATLAB-style string/value pairs:

      lines = plt.plot(x1, y1, x2, y2)
      # use keyword args
      plt.setp(lines, color='r', linewidth=2.0)
      # or MATLAB style string value pairs
      plt.setp(lines, 'color', 'r', 'linewidth', 2.0)

Here are the available `Line2D` properties.

Property | Value Type
--- | ---
alpha | float
animated | [True | False]
antialiased or aa | [True | False]
clip_box | a matplotlib.transform.Bbox instance
clip_on | [True | False]
clip_path | a Path instance and a Transform instance, a Patch
color or c | any matplotlib color
contains | the hit testing function
dash_capstyle | [`'butt'` | `'round'` | `'projecting'`]
dash_joinstyle | [`'miter'` | `'round'` | `'bevel'`]
dashes | sequence of on/off ink in points
data | (np.array xdata, np.array ydata)
figure | a matplotlib.figure.Figure instance
label | any string
linestyle or ls | [ `'-'` | `'--'` | `'-.'` | `':'` | `'steps'` | …]
linewidth or lw | float value in points
lod | [True | False]
marker | [ `'+'` | `','` | `'.'` | `'1'` | `'2'` | `'3'` | `'4'` ]
markeredgecolor or mec | any matplotlib color
markeredgewidth or mew | float value in points
markerfacecolor or mfc | any matplotlib color
markersize or ms | float
markevery | [ None | integer | (startind, stride) ]
picker | used in interactive line selection
pickradius | the line pick selection radius
solid_capstyle | [`'butt'` | `'round'` | `'projecting'`]
solid_joinstyle | [`'miter'` | `'round'` | `'bevel'`]
transform | a matplotlib.transforms.Transform instance
visible | [True | False]
xdata | np.array
ydata | np.array
zorder | any number

To get a list of settable line properties, call the `setp()` function with a line or lines as argument

    In [69]: lines = plt.plot([1, 2, 3])

    In [70]: plt.setp(lines)
      alpha: float
      animated: [True | False]
      antialiased or aa: [True | False]
      ...snip

## Working with multiple figures and axes

MATLAB, and `pyplot`, have the concept of the current figure and the current axes. All plotting commands apply to the current axes. The function `gca()` returns the current axes (a `matplotlib.axes.Axes` instance), and `gcf()` returns the current figure (`matplotlib.figure.Figure` instance). Normally, you don’t have to worry about this, because it is all taken care of behind the scenes. Below is a script to create two subplots.

    def f(t):
        return np.exp(-t) * np.cos(2*np.pi*t)

    t1 = np.arange(0.0, 5.0, 0.1)
    t2 = np.arange(0.0, 5.0, 0.02)

    plt.figure(1)
    plt.subplot(211)
    plt.plot(t1, f(t1), 'bo', t2, f(t2), 'k')

    plt.subplot(212)
    plt.plot(t2, np.cos(2*np.pi*t2), 'r--')
    plt.show()

The `figure()` command here is optional because `figure(1)` will be created by default, just as a `subplot(111)` will be created by default if you don’t manually specify any axes. The `subplot()` command specifies `numrows, numcols, fignum` where `fignum` ranges from 1 to `numrows*numcols`. The commas in the `subplot` command are optional if `numrows*numcols<10`. So `subplot(211)` is identical to `subplot(2, 1, 1)`.

You can create an arbitrary number of subplots and axes. If you want to place an axes manually, i.e., not on a rectangular grid, use the `axes()` command, which allows you to specify the location as `axes([left, bottom, width, height])` where all values are in fractional (0 to 1) coordinates. See Axes Demo for an example of placing axes manually and Subplot Demo for an example with lots of subplots.

You can create multiple figures by using multiple `figure()` calls with an increasing figure number. Of course, each figure can contain as many axes and subplots as your heart desires:

    import matplotlib.pyplot as plt
    plt.figure(1)                # the first figure
    plt.subplot(211)             # the first subplot in the first figure
    plt.plot([1, 2, 3])
    plt.subplot(212)             # the second subplot in the first figure
    plt.plot([4, 5, 6])

    plt.figure(2)                # a second figure
    plt.plot([4, 5, 6])          # creates a subplot(111) by default

    plt.figure(1)                # figure 1 current; subplot(212) still current
    plt.subplot(211)             # make subplot(211) in figure1 current
    plt.title('Easy as 1, 2, 3') # subplot 211 title

You can clear the current figure with `clf()` and the current axes with `cla()`. If you find it annoying that states (specifically the current image, figure and axes) are being maintained for you behind the scenes, don’t despair: this is just a thin stateful wrapper around an object oriented API, which you can use instead (see Artist tutorial)

If you are making lots of figures, you need to be aware of one more thing: the memory required for a figure is not completely released until the figure is explicitly closed with `close()`. Deleting all references to the figure, and/or using the window manager to kill the window in which the figure appears on the screen, is not enough, because pyplot maintains internal references until `close()` is called.

## Working with text

The `text()` command can be used to add text in an arbitrary location, and the `xlabel()`, `ylabel()` and `title()` are used to add text in the indicated locations (see Text introduction for a more detailed example)

    mu, sigma = 100, 15
    x = mu + sigma * np.random.randn(10000)

    # the histogram of the data
    n, bins, patches = plt.hist(x, 50, normed=1, facecolor='g', alpha=0.75)

    plt.xlabel('Smarts')
    plt.ylabel('Probability')
    plt.title('Histogram of IQ')
    plt.text(60, .025, r'$\mu=100,\ \sigma=15$')
    plt.axis([40, 160, 0, 0.03])
    plt.grid(True)
    plt.show()

All of the `text()` commands return an `matplotlib.text.Text` instance. Just as with with lines above, you can customize the properties by passing keyword arguments into the text functions or using `setp()`:

    t = plt.xlabel('my data', fontsize=14, color='red')

These properties are covered in more detail in Text properties and layout.

### Using mathematical expressions in text

matplotlib accepts TeX equation expressions in any text expression. For example to write the expression in the title, you can write a TeX expression surrounded by dollar signs:

    plt.title(r'$\sigma_i=15$')

The `r` preceding the title string is important – it signifies that the string is a raw string and not to treat backslashes as python escapes. matplotlib has a built-in TeX expression parser and layout engine, and ships its own math fonts – for details see Writing mathematical expressions. Thus you can use mathematical text across platforms without requiring a TeX installation. For those who have LaTeX and dvipng installed, you can also use LaTeX to format your text and incorporate the output directly into your display figures or saved postscript – see Text rendering With LaTeX.

### Annotating text

The uses of the basic `text()` command above place text at an arbitrary position on the Axes. A common use for text is to annotate some feature of the plot, and the `annotate()` method provides helper functionality to make annotations easy. In an annotation, there are two points to consider: the location being annotated represented by the argument `xy` and the location of the text `xytext`. Both of these arguments are `(x,y)` tuples.

    ax = plt.subplot(111)

    t = np.arange(0.0, 5.0, 0.01)
    s = np.cos(2*np.pi*t)
    line, = plt.plot(t, s, lw=2)

    plt.annotate('local max', xy=(2, 1), xytext=(3, 1.5),
                 arrowprops=dict(facecolor='black', shrink=0.05),
                 )

    plt.ylim(-2, 2)
    plt.show()

In this basic example, both the `xy` (arrow tip) and `xytext` locations (text location) are in data coordinates. There are a variety of other coordinate systems one can choose – see Basic annotation and Advanced Annotation for details. More examples can be found in Annotating Plots.

## Logarithmic and other nonlinear axes

`matplotlib.pyplot` supports not only linear axis scales, but also logarithmic and logit scales. This is commonly used if data spans many orders of magnitude. Changing the scale of an axis is easy:

> plt.xscale(‘log’)

An example of four plots with the same data and different scales for the y axis is shown below.

    from matplotlib.ticker import NullFormatter  # useful for `logit` scale

    # Fixing random state for reproducibility
    np.random.seed(19680801)

    # make up some data in the interval ]0, 1[
    y = np.random.normal(loc=0.5, scale=0.4, size=1000)
    y = y[(y > 0) & (y < 1)]
    y.sort()
    x = np.arange(len(y))

    # plot with various axes scales
    plt.figure(1)

    # linear
    plt.subplot(221)
    plt.plot(x, y)
    plt.yscale('linear')
    plt.title('linear')
    plt.grid(True)

    # log
    plt.subplot(222)
    plt.plot(x, y)
    plt.yscale('log')
    plt.title('log')
    plt.grid(True)

    # symmetric log
    plt.subplot(223)
    plt.plot(x, y - y.mean())
    plt.yscale('symlog', linthreshy=0.01)
    plt.title('symlog')
    plt.grid(True)

    # logit
    plt.subplot(224)
    plt.plot(x, y)
    plt.yscale('logit')
    plt.title('logit')
    plt.grid(True)
    # Format the minor tick labels of the y-axis into empty strings with
    # `NullFormatter`, to avoid cumbering the axis with too many labels.
    plt.gca().yaxis.set_minor_formatter(NullFormatter())
    # Adjust the subplot layout, because the logit one may take more space
    # than usual, due to y-tick labels like "1 - 10^{-3}"
    plt.subplots_adjust(top=0.92, bottom=0.08, left=0.10, right=0.95, hspace=0.25,
                        wspace=0.35)

    plt.show()

It is also possible to add your own scale, see Developer’s guide for creating scales and transformations for details.

Total running time of the script: ( 0 minutes 0.544 seconds)

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