What is the X and Y Axis?
The X and Y axis form the basis of most graphs. These two perpendicular lines define the coordinate plane. X and Y values can specify any point on this plane using the Cartesian coordinate system.
In this system, the axes are the following:
- X Axis: Horizontal, also known as the abscissa.
- Y Axis: Vertical, also known as the ordinate.
The grid below displays the X and Y axes. Coordinates are depicted using parentheses—(X, Y). For example, the red data point is at (3, 5).
The X and Y axes have the following properties in the Cartesian coordinate system:
- The origin is always at (0, 0).
- Positive X values are to the right of the Y axis.
- Positive Y values are above the X axis.
When you are working with a line, the Y-intercept is where the line crosses the Y axis. It always has an X value = 0, while the Y value is the point where the line crosses the Y Axis. For example, in the graph below, the Y-intercept for the green line is (0, 4), and for the red line it is (0, -2).
That’s the basic usage for the X and Y axis. Now, let’s learn how different graphs use them to gain a better understanding of what they want to convey!
Learn more about using equations to draw lines in my Guide to the Slope Intercept Form of Linear Equations.
X and Y Axis in Other Graphs
In graphing and statistics, the X and Y axis each displays different kinds of information depending on the type of graph. And there are conventions about what should appear on each axis.
As we go through some common graphs, pay particular attention to the axes. Notice how each graph type has both an X and Y axis but uses them differently. That’ll help you understand the meaning behind each graph.
Scatterplots are visually and functionally the most like the Cartesian coordinate system, except they don’t display the gridlines. Use these graphs to plot pairs of X and Y data points. They’re perfect for visualizing the relationship between two continuous variables.
For example, in the graph below, the X axis represents height, and the Y axis denotes weight. Each dot’s (X, Y) coordinate represents an individual’s height and weight combination. You can see how weight tends to increase for taller people.
A statistical convention is that when you have a pair of variables and one variable explains the changes in the other variable, you include the explanatory variable on the X axis and the outcome variable on the Y axis.
Scatterplots can superimpose a fitted regression line for simple regression models. In these graphs, the Y axis displays the dependent variable, while the X axis displays the independent or predictor variable. Y-intercepts for regression lines are one of the parameters that the model estimates.
Histograms display how frequently continuous data fall within ranges of values, known as bins. These graphs are fantastic for understanding a distribution’s center, spread, and shape.
The most common type of histogram has vertical bars, as shown below. In this orientation, the X axis represents the continuous variable. The Y axis represents counts, percentages, or probabilities of the observations falling within each bin.
The histogram below displays body fat percentages horizontally and vertically depicts the number of times (i.e., frequency) observations fall within each bin. Most people have body fat percentages close to 25%, but some are notably higher.
Occasionally, you’ll see rotated histograms that switch the axes. Those histograms have horizontal bars.
Learn more about Histograms.
Time Series Plots
Time series plots show how a variable changes over time. Most commonly, the X axis displays the time, while the Y axis displays the outcome variable that you are tracking. By connecting the data points, time series plots emphasize how the value changes as time passes.
For example, the time series plot below tracks the number of COVID cases vertically, while it shows time in days horizontally. There appears to be a COVID surge near the end of the dataset.
Learn more about Time Series Plots.
Bar charts help us understand categorical and other discrete variables. On these graphs, the X axis displays categories or discrete values. The Y axis represents counts or a summary value, such as the average.
For example, the bar chart below displays the categories of delivery statuses and peak/off peak times horizontally. The Y axis displays the count of times that each one occurs. The proportion of late deliveries increases during peak times.
Learn more about Bar Charts.
Box plots allow you to compare the center and spread of continuous data across groups or categories. Typically, these graphs display the groups along the X axis and the continuous outcome variable on the Y axis. However, like the histogram, analysts occasionally rotate these graphs.
For example, the box plot below displays teaching method horizontally and the test scores vertically. Teaching method 4 has the highest median score.
Learn more about Box Plots.
By learning how a graph uses these two axes, you’ll understand what it wants to convey.