Your first graph

What you will be able to do

After this lesson you can

• draw a line graph from two arrays with plt.plot,
• add a title, axis labels with units, a legend and a grid,
• change the markers, the line style and the colour, and
• put two lines on one graph and read it.

Why a graph?

Here are six numbers: 0, 150, 330, 480, 650, 800. Do they tell you anything? Now see them on a graph: the line rises, and you can see how fast, and whether the speed changes. A graph turns a table of numbers into a picture that a person understands in a second. Every science report has graphs, and they are the first thing that a reader looks at.

The library for graphs in Python is matplotlib. You use its part called pyplot, and by custom you import it as plt:

On this site, matplotlib is ready. The first time that your code uses it, the site needs a few seconds to load it. The graph appears under the output of your program.

The first graph

Predict first. Will the line go up or down? Will it be a straight line? Then run it.

The line rises from 0 to 800 metres. In the first minute the cyclist goes 150 m, and in the last minute 150 m again. Between them there is a faster minute, from 2 to 3 minutes, when the cyclist rode 180 m.

Markers, styles, legend and grid

The last line of the program prints a number. Asha is 260 m ahead after 5 minutes. On the graph, this is the gap between the two lines at the right end.

A good graph says what each axis is and in which unit. Without labels, nobody can read it, however nice it looks.

What a good graph has

• A title that says what the graph is about.
• Labels on both axes, with units.
• A legend when there is more than one line.
• Markers when the points are measurements, so that the reader can see where the data is.
• Enough room: the lines must not be squeezed into a corner.

Making these a habit is a large part of the skill.

Common mistakes

• Two arrays of different lengths. plt.plot([1, 2, 3], [4, 5]) gives a ValueError: x and y must have the same first dimension.
• Giving the arrays in the wrong order. The first is x, the second is y.
• A legend without labels. plt.legend() shows nothing useful if the lines have no label.
• Forgetting the units. A reader cannot guess them.
• Joining measurements with a smooth line that the data does not support. Use markers.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Plot the cyclists. You write a function that draws the graph, and the checker looks at the lines and the labels.
2. Build. Open the problem Read the graph and the problem Fix the plot.

Remember these

• plt.plot(x, y) joins the points. x comes first.
• plt.title, plt.xlabel and plt.ylabel name the graph. Write the unit.
• A format such as "o-" sets the markers and the line. Use label and plt.legend() for several lines.
• Every plt.plot adds a line to the same picture.
• x and y must have the same length.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What does the format "s--" mean?

2. What must you do to see the labels of the lines?

3. What happens with plt.plot([1, 2, 3], [4, 5])?

4. Which array is on the horizontal axis in plt.plot(a, b)?