The Lesson

The y-axis is the vertical axis on a graph. The image below shows what we mean by the y-axis:

Note: Only part of the y-axis is shown. It can extend forever up and down.

How to Use the Y-Axis

Imagine you wanted to describe the position of a point on a graph. You would need to know how far across a point is and how far up a point is. The y-axis lets us measure how far up a point is. The y-axis is labelled with numbers (0, 1, 2, 3...) so you can measure how far up the point is. This gives the y-coordinate of a point, using Cartesian coordinates.

Question

What is the y-coordinate of the point below?

Step-by-Step:

1

See how far up the y-axis the point is. It can help to draw a line (in your minds eye or otherwise) from the point, straight across to the y-axis.

Answer:

We can see that the point is 4 units up the y-axis. The point has a y coordinate of 4.

The Y-Axis Goes On Forever, Up and Down

In the images above, the y-axis starts at 0 and goes up to 5. This can give the wrong impression. In fact, the y-axis can continue going up (past 100, past 1000, in fact forever). The y-axis also goes below 0. It is labelled -1, -2, -3. It can also extend forever downwards.

In practice, only draw the parts of the y-axis you need to show.

The Y-Axis as a Number Line

The y-axis is a number line that extends vertically.

The Equation of the Y-Axis

The equation of the y-axis is:

It is an equation of a line. This equation is true because the y-axis crosses the x-axis when x = 0. The x-coordinate of all points on the y-axis are 0

Which Axis Is Which?

The x is a cross - so the x-axis goes across! The y-axis must go up.

Only Draw the Parts of the Y-Axis You Need

A graph can be used to plot, points, lines and curves. You don't want a lot of blank space when you draw a graph. Only draw the parts of the y-axis you need. The curve below only takes up a small part of the y-axis. There's no need to include the part of the y-axis below the x-axis.

Scaling a Graph

A graph can be used to plot, points, lines and curves. A line or curve might extend a long way vertically. In the image below, the line extends past 100 on the y-axis.

You won't want to label every unit 1, 2, 3... all the way up to 100. You might rather label in tens (10, 20, 30.. up to 100). Or perhaps in twenties (20, 40...) or fifties (50, 100). Choose an appropriate scale when drawing a graph.