Time for action – plotting a polynomial function

To illustrate how plotting works, let's display some polynomial graphs. We will use the NumPy polynomial function poly1d() to create a polynomial.

  1. Take the standard input values as polynomial coefficients. Use the NumPy poly1d() function to create a polynomial:
    func = np.poly1d(np.array([1, 2, 3, 4]).astype(float))
  2. Create the x values with the NumPy the linspace() function. Use the range -10 to 10 and create 30 even spaced values:
    x = np.linspace(-10, 10, 30)
  3. Calculate the polynomial values using the polynomial we created in the first step:
    y = func(x)
  4. Call the plot() function; this does not immediately display the graph:
    plt.plot(x, y)
  5. Add a label to the x axis with the xlabel() function:
    plt.xlabel('x')
  6. Add a label to the y axis with the ylabel() function:
    plt.ylabel('y(x)')
  7. Call the show() function to display the graph:
    plt.show()

    The following is a plot with polynomial coefficients 1, 2, 3, and 4:

    Time for action – plotting a polynomial function

What just happened?

We displayed a polynomial graph on our screen. We added labels to the x and y axes (see polyplot.py):

import numpy as np
import matplotlib.pyplot as plt

func = np.poly1d(np.array([1, 2, 3, 4]).astype(float))
x = np.linspace(-10, 10, 30)
y = func(x)

plt.plot(x, y)
plt.xlabel('x')
plt.ylabel('y(x)')
plt.show()

Pop quiz – the plot() function

Q1. What does the plot() function do?

  1. It displays two-dimensional plots on screen.
  2. It saves an image of a two-dimensional plot in a file.
  3. It does both a and b.
  4. It does neither a, b, or c.
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