Program 4: A Line Chart from (2, 5) to (9, 10)

Write a program to display a line chart from (2, 5) to (9, 10).

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1Aim

Write a program to display a line chart from (2, 5) to (9, 10).

Python 3Matplotlib

2Program

line_chart.py

# Program 4: Display a line chart from (2, 5) to (9, 10)

import matplotlib.pyplot as plt

x = [2, 9]       # x values of the two points
y = [5, 10]      # y values of the two points

plt.plot(x, y, color="green", marker="o", linewidth=2)
plt.title("Line chart from (2, 5) to (9, 10)")
plt.xlabel("x axis")
plt.ylabel("y axis")
plt.grid(True)
plt.show()

# The slope of the line: how much y rises for each step in x.
slope = (y[1] - y[0]) / (x[1] - x[0])
print("The line rises", round(slope, 3), "in y for every 1 in x")

3Output

The output of a real run, exactly as it appeared.

Output

The line rises 0.714 in y for every 1 in x

the window opened by plt.show()

4How it works

x = [2, 9] and y = [5, 10]

Matplotlib takes all the x values in one list and all the y values in another. The two points are (2, 5) and (9, 10).

plt.plot(x, y, color="green", marker="o")

Draws a line through the points; marker="o" puts a dot on each point.

plt.title / plt.xlabel / plt.ylabel

A chart without a title and axis labels loses marks — always add them.

plt.show()

Displays the chart. Nothing appears on screen without it.

5Try this

Change the colour to red and the line style to dashed (linestyle="--").
Add a third point, (12, 4).
Draw a second line from (2, 10) to (9, 5) on the same chart.

6Viva questions

▸Which function draws a line chart?

plt.plot().

▸How are the points (2, 5) and (9, 10) given to plt.plot()?

As two lists: x = [2, 9] and y = [5, 10].

▸What does plt.show() do?

It displays the chart.

Tip

In your practical file, write the aim, the program and the output for each program — run it yourself and copy your own output. See The Practical File for how to set the file out.

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