Matplotlib


Visualization with Matplotlib

Learning Objectives: Learn how to make basic plots using Matplotlib's pylab API and how to use the Matplotlib documentation.

This notebook focuses only on the Matplotlib API, rather that the broader question of how you can use this API to make effective and beautiful visualizations.

Imports

The following imports should be used in all of your notebooks where Matplotlib in used:


In [1]:
%matplotlib inline
import matplotlib.pyplot as plt
import numpy as np

Overview

The following conceptual organization is simplified and adapted from Benjamin Root's AnatomyOfMatplotlib tutorial.

Figures and Axes

  • In Matplotlib a single visualization is a Figure.
  • A Figure can have multiple areas, called subplots. Each subplot is an Axes.
  • If you don't create a Figure and Axes yourself, Matplotlib will automatically create one for you.
  • All plotting commands apply to the current Figure and Axes.

The following functions can be used to create and manage Figure and Axes objects.

Function Description
figure Creates a new Figure
gca Get the current Axes instance
savefig Save the current Figure to a file
sca Set the current Axes instance
subplot Create a new subplot Axes for the current Figure
subplots Create a new Figure and a grid of subplots Axes

Plotting Functions

Once you have created a Figure and one or more Axes objects, you can use the following function to put data onto that Axes.

Function Description
bar Make a bar plot
barh Make a horizontal bar plot
boxplot Make a box and whisker plot
contour Plot contours
contourf Plot filled contours
hist Plot a histogram
hist2d Make a 2D histogram plot
imshow Display an image on the axes
matshow Display an array as a matrix
pcolor Create a pseudocolor plot of a 2-D array
pcolormesh Plot a quadrilateral mesh
plot Plot lines and/or markers
plot_date Plot with data with dates
polar Make a polar plot
scatter Make a scatter plot of x vs y

Plot modifiers

You can then use the following functions to modify your visualization.

Function Description
annotate Create an annotation: a piece of text referring to a data point
box Turn the Axes box on or off
clabel Label a contour plot
colorbar Add a colorbar to a plot
grid Turn the Axes grids on or off
legend Place a legend on the current Axes
loglog Make a plot with log scaling on both the x and y axis
semilogx Make a plot with log scaling on the x axis
semilogy Make a plot with log scaling on the y axis
subplots_adjust Tune the subplot layout
tick_params Change the appearance of ticks and tick labels
ticklabel_format Change the ScalarFormatter used by default for linear axes
tight_layout Automatically adjust subplot parameters to give specified padding
text Add text to the axes
title Set a title of the current axes
xkcd Turns on XKCD sketch-style drawing mode
xlabel Set the x axis label of the current axis
xlim Get or set the x limits of the current axes
xticks Get or set the x-limits of the current tick locations and labels
ylabel Set the y axis label of the current axis
ylim Get or set the y-limits of the current axes
yticks Get or set the y-limits of the current tick locations and labels

Basic plotting

For now, we will work with basic line plots (plt.plot) to show how the Matplotlib pylab plotting API works. In this case, we don't create a Figure so Matplotlib does that automatically.


In [2]:
t = np.linspace(0, 10.0, 100)
plt.plot(t, np.sin(t))
plt.xlabel('Time')
plt.ylabel('Signal')
plt.title('My Plot'); # supress text output


Out[2]:
<matplotlib.text.Text at 0x7f520d1912e8>

Basic plot modification

With a third argument you can provide the series color and line/marker style. Here we create a Figure object and modify its size.


In [3]:
f = plt.figure(figsize=(9,6)) # 9" x 6", default is 8" x 5.5"

plt.plot(t, np.sin(t), 'r.');
plt.xlabel('x')
plt.ylabel('y')


Out[3]:
<matplotlib.text.Text at 0x7f520d185cf8>

Here is a list of the single character color strings:

    b: blue
    g: green
    r: red
    c: cyan
    m: magenta
    y: yellow
    k: black
    w: white

The following will show all of the line and marker styles:


In [4]:
from matplotlib import lines
lines.lineStyles.keys()


Out[4]:
dict_keys(['', 'None', ' ', '-.', ':', '-', '--'])

In [5]:
from matplotlib import markers
markers.MarkerStyle.markers.keys()


Out[5]:
dict_keys(['<', '.', '>', 3, 4, 5, 0, '2', 'd', 2, 'x', 'None', '4', 'D', '1', 's', 7, '_', '|', 'o', '', ',', 1, 6, '8', '^', 'h', 'v', ' ', 'p', '3', None, '*', '+', 'H'])

To change the plot's limits, use xlim and ylim:


In [6]:
plt.plot(t, np.sin(t)*np.exp(-0.1*t),'bo')
plt.xlim(-1.0, 11.0)
plt.ylim(-1.0, 1.0)


Out[6]:
(-1.0, 1.0)

You can change the ticks along a given axis by using xticks, yticks and tick_params:


In [7]:
plt.plot(t, np.sin(t)*np.exp(-0.1*t),'bo')
plt.xlim(0.0, 10.0)
plt.ylim(-1.0, 1.0)
plt.xticks([0,5,10], ['zero','five','10'])
plt.tick_params(axis='y', direction='inout', length=10)


Box and grid

You can enable a grid or disable the box. Notice that the ticks and tick labels remain.


In [45]:
plt.plot(np.random.rand(100), 'b-')
plt.grid(True)
plt.box(False)


Multiple series

Multiple calls to a plotting function will all target the current Axes:


In [15]:
plt.plot(t, np.sin(t), label='sin(t)')
plt.plot(t, np.cos(t), label='cos(t)')
plt.xlabel('t')
plt.ylabel('Signal(t)')
plt.ylim(-1.5, 1.5)
plt.xlim(right=12.0)
plt.legend()


Out[15]:
<matplotlib.legend.Legend at 0x7f555842ef98>

Subplots

Subplots allow you to create a grid of plots in a single figure. There will be an Axes associated with each subplot and only one Axes can be active at a time.

The first way you can create subplots is to use the subplot function, which creates and activates a new Axes for the active Figure:


In [17]:
plt.subplot(2,1,1) # 2 rows x 1 col, plot 1
plt.plot(t, np.exp(0.1*t))
plt.ylabel('Exponential')

plt.subplot(2,1,2) # 2 rows x 1 col, plot 2
plt.plot(t, t**2)
plt.ylabel('Quadratic')
plt.xlabel('x')

plt.tight_layout()


In many cases, it is easier to use the subplots function, which creates a new Figure along with an array of Axes objects that can be indexed in a rational manner:


In [23]:
f, ax = plt.subplots(2, 2)

for i in range(2):
    for j in range(2):
        plt.sca(ax[i,j])
        plt.plot(np.random.rand(20))
        plt.xlabel('x')
        plt.ylabel('y')

plt.tight_layout()


The subplots function also makes it easy to pass arguments to Figure and to share axes:


In [33]:
f, ax = plt.subplots(2, 2, sharex=True, sharey=True, figsize=(6,6))

for i in range(2):
    for j in range(2):
        plt.sca(ax[i,j])
        plt.plot(np.random.rand(20))
        if i==1:
            plt.xlabel('x')
        if j==0:
            plt.ylabel('y')

plt.tight_layout()


More marker and line styling

All plot commands, including plot, accept keyword arguments that can be used to style the lines in more detail. Fro more information see:


In [11]:
plt.plot(t, np.sin(t), marker='o', color='darkblue',
         linestyle='--', alpha=0.3, markersize=10)


Out[11]:
[<matplotlib.lines.Line2D at 0x109200d50>]

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