How

How to draw a scatter plot

We use the matplotlib.pyplot.scatter tool. This takes two iterables as arguments: one for the first dimension and one for the other dimension.

Tip

import matplotlib.pyplot as plt

plt.figure()
plt.scatter(x, y)

Here is an example:

import matplotlib.pyplot as plt
import numpy as np

x = np.array((1, 2, 3, 4, 5))
y = np.array((-1, -2, 4, -1, 7))
plt.figure()
plt.scatter(x, y);

How to plot a function

We use the matplotlib.pyplot.plot tool. Similarly to matplotlib.pyplot.scatter this takes two iterables. It plots a line plot. To plot a function we use this an generate the required data.

Tip

import matplotlib.pyplot as plt

plt.figure()
plt.plot(x, y)

Here is an example of how we would plot \(f(x)=x ^ 2 + 3x + 1\):

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-40, 40, 1000)
y = x ** 2 + 3 * x + 1
plt.figure()
plt.plot(x, y);

How to combine plots

To combine plots they can be included one after the other after the matplotlib.pyplot.figure call.

Tip

import matplotlib.pyplot as plt

plt.figure()
plt.plot(x, y)
plt.scatter(x, y)

For example the following will draw two different functions \(f(x) = cos(x)\) and \(g(x)=sin(x)\) (we include an argument to plot \(g\) in red):

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-10, 10, 1000)
f = np.cos(x)
g = np.sin(x)
plt.figure()
plt.plot(x, f)
plt.plot(x, g, color="red");

How to add a title

To add a title we use matplotlib.pyplot.title which takes a string.

Tip

import matplotlib.pyplot as plt

plt.figure()
plt.title(label)

Here we add a title to a plot of \(cos(x)\):

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-10, 10, 1000)
f = np.cos(x)
plt.figure()
plt.plot(x, f)
plt.title("Plot of cos(x)");

We can include LaTeX inside of $ signs to render mathematics:

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-10, 10, 1000)
f = np.cos(x)
plt.figure()
plt.plot(x, f)
plt.title("Plot of $\cos(x)$");

How to add axes labels

To add labels we use matplotlib.pyplot.xlabel or matplotlib.pyplot.ylabel. These both take a string.

Tip

import matplotlib.pyplot as plt

plt.figure()
plt.xlabel(label)
plt.ylabel(label)

Here is an example:

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-10, 10, 1000)
f = np.cos(x)
plt.figure()
plt.plot(x, f)
plt.xlabel("$x$")
plt.ylabel("$y$");

How to add a legend

When plotting multiple plots it can be helpful to include a legend. We do this by passing a label to the specific plot as a string. The legend then also need to be specified, that is done with matpltlib.pyplot.legend.

Tip

import matplotlib.pyplot as plt

plt.figure()
plt.scatter(x, y, label)
plt.legend()

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-10, 10, 1000)
f = np.cos(x)
g = np.sin(x)
plt.figure()
plt.plot(x, f, label="$cos(x)$")
plt.plot(x, g, label="$sin(x)$", color="red")
plt.legend();

How to save a plot

To save a plot to file we use the matplotlib.pyplot.savefig tool that takes a string as an argument for the name of the file.

Tip

import matplotlib.pyplot as plt

plt.figure()
plt.scatter(x, y, label)
plt.savefig(fname)

fname is a string that represents the name of the file.

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-10, 10, 1000)
f = np.cos(x)
g = np.sin(x)
plt.figure()
plt.plot(x, f, label="$cos(x)$")
plt.plot(x, g, label="$sin(x)$", color="red")
plt.legend();

Tip

import matplotlib.pyplot as plt

plt.figure()
plt.scatter(x, y, label)
plt.legend()

import matplotlib.pyplot as plt
import numpy as np

x = np.array((1, 2, 3, 4, 5))
y = np.array((-1, -2, 4, -1, 7))
plt.figure()
plt.scatter(x, y)
plt.savefig("plot.pdf")

You can pass various extensions as the name of the file (.pdf, png, .svg). This specifies the format of the file. In general .pdf is the format of the file that ensures the highest quality.